Safe Haskell | None |
---|
- ($) :: (a -> b) -> a -> b
- ($!) :: (a -> b) -> a -> b
- (&&) :: Bool -> Bool -> Bool
- (||) :: Bool -> Bool -> Bool
- (.) :: Category cat => forall b c a. cat b c -> cat a b -> cat a c
- not :: Bool -> Bool
- otherwise :: Bool
- fst :: (a, b) -> a
- snd :: (a, b) -> b
- id :: Category cat => forall a. cat a a
- maybe :: b -> (a -> b) -> Maybe a -> b
- either :: (a -> c) -> (b -> c) -> Either a b -> c
- flip :: (a -> b -> c) -> b -> a -> c
- const :: a -> b -> a
- error :: [Char] -> a
- putStrLn :: Text -> IO ()
- getArgs :: IO [Text]
- odd :: Integral a => a -> Bool
- even :: Integral a => a -> Bool
- uncurry :: (a -> b -> c) -> (a, b) -> c
- curry :: ((a, b) -> c) -> a -> b -> c
- swap :: (a, b) -> (b, a)
- until :: (a -> Bool) -> (a -> a) -> a -> a
- asTypeOf :: a -> a -> a
- undefined :: a
- seq :: a -> b -> b
- class Eq a => Ord a where
- class Eq a where
- class Bounded a where
- class Enum a where
- succ :: a -> a
- pred :: a -> a
- toEnum :: Int -> a
- fromEnum :: a -> Int
- enumFrom :: a -> [a]
- enumFromThen :: a -> a -> [a]
- enumFromTo :: a -> a -> [a]
- enumFromThenTo :: a -> a -> a -> [a]
- class Show a
- class Read a
- class Functor f where
- class Monad m where
- (=<<) :: Monad m => (a -> m b) -> m a -> m b
- class IsString a where
- fromString :: String -> a
- class Num a where
- class (Num a, Ord a) => Real a where
- toRational :: a -> Rational
- class (Real a, Enum a) => Integral a where
- class Num a => Fractional a where
- (/) :: a -> a -> a
- recip :: a -> a
- fromRational :: Rational -> a
- class Fractional a => Floating a where
- class (Real a, Fractional a) => RealFrac a where
- class (RealFrac a, Floating a) => RealFloat a where
- floatRadix :: a -> Integer
- floatDigits :: a -> Int
- floatRange :: a -> (Int, Int)
- decodeFloat :: a -> (Integer, Int)
- encodeFloat :: Integer -> Int -> a
- exponent :: a -> Int
- significand :: a -> a
- scaleFloat :: Int -> a -> a
- isNaN :: a -> Bool
- isInfinite :: a -> Bool
- isDenormalized :: a -> Bool
- isNegativeZero :: a -> Bool
- isIEEE :: a -> Bool
- atan2 :: a -> a -> a
- data Maybe a
- data Ordering
- data Bool
- data Char
- data IO a
- data Either a b
- data ByteString
- type LByteString = ByteString
- data Text
- type LText = Text
- data Map k a
- data HashMap k v
- data Set a
- data HashSet a
- data Vector a
- type UVector = Vector
- class (Vector Vector a, MVector MVector a) => Unbox a
- class Hashable a
- data Word
- data Word8
- data Word32
- data Word64
- data Int
- data Int32
- data Int64
- data Integer
- type Rational = Ratio Integer
- data Float
- data Double
- (^) :: (Num a, Integral b) => a -> b -> a
- (^^) :: (Fractional a, Integral b) => a -> b -> a
- subtract :: Num a => a -> a -> a
- fromIntegral :: (Integral a, Num b) => a -> b
- realToFrac :: (Real a, Fractional b) => a -> b
- class Monoid a where
- (<>) :: Monoid m => m -> m -> m
- first :: Arrow a => forall b c d. a b c -> a (b, d) (c, d)
- second :: Arrow a => forall b c d. a b c -> a (d, b) (d, c)
- (***) :: Arrow a => forall b c b' c'. a b c -> a b' c' -> a (b, b') (c, c')
- (&&&) :: Arrow a => forall b c c'. a b c -> a b c' -> a b (c, c')
- mapMaybe :: (a -> Maybe b) -> [a] -> [b]
- catMaybes :: [Maybe a] -> [a]
- fromMaybe :: a -> Maybe a -> a
- isJust :: Maybe a -> Bool
- isNothing :: Maybe a -> Bool
- listToMaybe :: [a] -> Maybe a
- maybeToList :: Maybe a -> [a]
- partitionEithers :: [Either a b] -> ([a], [b])
- lefts :: [Either a b] -> [a]
- rights :: [Either a b] -> [b]
- on :: (b -> b -> c) -> (a -> b) -> a -> a -> c
- comparing :: Ord a => (b -> a) -> b -> b -> Ordering
- equating :: Eq a => (b -> a) -> b -> b -> Bool
- class Functor f => Applicative f where
- (<$>) :: Functor f => (a -> b) -> f a -> f b
- (>=>) :: Monad m => (a -> m b) -> (b -> m c) -> a -> m c
- lift :: MonadTrans t => forall m a. Monad m => m a -> t m a
- class Monad m => MonadIO m where
- liftIO :: MonadIO m => forall a. IO a -> m a
- class (Typeable e, Show e) => Exception e where
- toException :: e -> SomeException
- fromException :: SomeException -> Maybe e
- class Typeable a where
- data SomeException
- data IOException
- throwIO :: (MonadBase IO m, Exception e) => e -> m a
- try :: (MonadBaseControl IO m, Exception e) => m a -> m (Either e a)
- catch :: (MonadBaseControl IO m, Exception e) => m a -> (e -> m a) -> m a
- bracket :: MonadBaseControl IO m => m a -> (a -> m b) -> (a -> m c) -> m c
- onException :: MonadBaseControl IO m => m a -> m b -> m a
- finally :: MonadBaseControl IO m => m a -> m b -> m a
- data FilePath
- (</>) :: FilePath -> FilePath -> FilePath
- (<.>) :: FilePath -> Text -> FilePath
- hasExtension :: FilePath -> Text -> Bool
- basename :: FilePath -> FilePath
- filename :: FilePath -> FilePath
- directory :: FilePath -> FilePath
- print :: Show a => a -> IO ()
- readArgs :: ArgumentTuple a => IO a
Standard
Operators
($) :: (a -> b) -> a -> b
Application operator. This operator is redundant, since ordinary
application (f x)
means the same as (f
. However, $
x)$
has
low, right-associative binding precedence, so it sometimes allows
parentheses to be omitted; for example:
f $ g $ h x = f (g (h x))
It is also useful in higher-order situations, such as
,
or map
($
0) xs
.
zipWith
($
) fs xs
Functions
fst :: (a, b) -> a
Extract the first component of a pair.
snd :: (a, b) -> b
Extract the second component of a pair.
flip :: (a -> b -> c) -> b -> a -> c
takes its (first) two arguments in the reverse order of flip
ff
.
const :: a -> b -> a
Constant function.
swap :: (a, b) -> (b, a)
Swap the components of a pair.
asTypeOf :: a -> a -> a
undefined :: a
seq :: a -> b -> b
Evaluates its first argument to head normal form, and then returns its second argument as the result.
Type classes
The Ord
class is used for totally ordered datatypes.
Instances of Ord
can be derived for any user-defined
datatype whose constituent types are in Ord
. The declared order
of the constructors in the data declaration determines the ordering
in derived Ord
instances. The Ordering
datatype allows a single
comparison to determine the precise ordering of two objects.
Minimal complete definition: either compare
or <=
.
Using compare
can be more efficient for complex types.
Ord Bool | |
Ord Char | |
Ord Double | |
Ord Float | |
Ord Int | |
Ord Int8 | |
Ord Int16 | |
Ord Int32 | |
Ord Int64 | |
Ord Integer | |
Ord Ordering | |
Ord Word | |
Ord Word8 | |
Ord Word16 | |
Ord Word32 | |
Ord Word64 | |
Ord () | |
Ord ThreadId | |
Ord BlockReason | |
Ord ThreadStatus | |
Ord AsyncException | |
Ord ArrayException | |
Ord ExitCode | |
Ord All | |
Ord Any | |
Ord Arity | |
Ord Fixity | |
Ord Associativity | |
Ord TypeRepKey | |
Ord ArithException | |
Ord TypeRep | |
Ord TyCon | |
Ord ByteString | |
Ord ByteString | |
Ord Text | |
Ord Root | |
Ord FilePath | |
Ord Text | |
(Eq [a], Ord a) => Ord [a] | |
(Eq (Ratio a), Integral a) => Ord (Ratio a) | |
(Eq (Dual a), Ord a) => Ord (Dual a) | |
(Eq (Sum a), Ord a) => Ord (Sum a) | |
(Eq (Product a), Ord a) => Ord (Product a) | |
(Eq (First a), Ord a) => Ord (First a) | |
(Eq (Last a), Ord a) => Ord (Last a) | |
(Eq (Down a), Ord a) => Ord (Down a) | |
(Eq (Maybe a), Ord a) => Ord (Maybe a) | |
(Eq (Set a), Ord a) => Ord (Set a) | |
(Eq (Vector a), Ord a) => Ord (Vector a) | |
(Eq (Vector a), Unbox a, Ord a) => Ord (Vector a) | |
(Eq (Vector a), Prim a, Ord a) => Ord (Vector a) | |
(Eq (Either a b), Ord a, Ord b) => Ord (Either a b) | |
(Eq (a, b), Ord a, Ord b) => Ord (a, b) | |
(Eq (Map k v), Ord k, Ord v) => Ord (Map k v) | |
(Eq (Stream Id a), Ord a) => Ord (Stream Id a) | |
(Eq (a, b, c), Ord a, Ord b, Ord c) => Ord (a, b, c) | |
(Eq (a, b, c, d), Ord a, Ord b, Ord c, Ord d) => Ord (a, b, c, d) | |
(Eq (a, b, c, d, e), Ord a, Ord b, Ord c, Ord d, Ord e) => Ord (a, b, c, d, e) | |
(Eq (a, b, c, d, e, f), Ord a, Ord b, Ord c, Ord d, Ord e, Ord f) => Ord (a, b, c, d, e, f) | |
(Eq (a, b, c, d, e, f, g), Ord a, Ord b, Ord c, Ord d, Ord e, Ord f, Ord g) => Ord (a, b, c, d, e, f, g) | |
(Eq (a, b, c, d, e, f, g, h), Ord a, Ord b, Ord c, Ord d, Ord e, Ord f, Ord g, Ord h) => Ord (a, b, c, d, e, f, g, h) | |
(Eq (a, b, c, d, e, f, g, h, i), Ord a, Ord b, Ord c, Ord d, Ord e, Ord f, Ord g, Ord h, Ord i) => Ord (a, b, c, d, e, f, g, h, i) | |
(Eq (a, b, c, d, e, f, g, h, i, j), Ord a, Ord b, Ord c, Ord d, Ord e, Ord f, Ord g, Ord h, Ord i, Ord j) => Ord (a, b, c, d, e, f, g, h, i, j) | |
(Eq (a, b, c, d, e, f, g, h, i, j, k), Ord a, Ord b, Ord c, Ord d, Ord e, Ord f, Ord g, Ord h, Ord i, Ord j, Ord k) => Ord (a, b, c, d, e, f, g, h, i, j, k) | |
(Eq (a, b, c, d, e, f, g, h, i, j, k, l), Ord a, Ord b, Ord c, Ord d, Ord e, Ord f, Ord g, Ord h, Ord i, Ord j, Ord k, Ord l) => Ord (a, b, c, d, e, f, g, h, i, j, k, l) | |
(Eq (a, b, c, d, e, f, g, h, i, j, k, l, m), Ord a, Ord b, Ord c, Ord d, Ord e, Ord f, Ord g, Ord h, Ord i, Ord j, Ord k, Ord l, Ord m) => Ord (a, b, c, d, e, f, g, h, i, j, k, l, m) | |
(Eq (a, b, c, d, e, f, g, h, i, j, k, l, m, n), Ord a, Ord b, Ord c, Ord d, Ord e, Ord f, Ord g, Ord h, Ord i, Ord j, Ord k, Ord l, Ord m, Ord n) => Ord (a, b, c, d, e, f, g, h, i, j, k, l, m, n) | |
(Eq (a, b, c, d, e, f, g, h, i, j, k, l, m, n, o), Ord a, Ord b, Ord c, Ord d, Ord e, Ord f, Ord g, Ord h, Ord i, Ord j, Ord k, Ord l, Ord m, Ord n, Ord o) => Ord (a, b, c, d, e, f, g, h, i, j, k, l, m, n, o) |
class Eq a where
The Eq
class defines equality (==
) and inequality (/=
).
All the basic datatypes exported by the Prelude are instances of Eq
,
and Eq
may be derived for any datatype whose constituents are also
instances of Eq
.
Eq Bool | |
Eq Char | |
Eq Double | |
Eq Float | |
Eq Int | |
Eq Int8 | |
Eq Int16 | |
Eq Int32 | |
Eq Int64 | |
Eq Integer | |
Eq Ordering | |
Eq Word | |
Eq Word8 | |
Eq Word16 | |
Eq Word32 | |
Eq Word64 | |
Eq () | |
Eq ThreadId | |
Eq BlockReason | |
Eq ThreadStatus | |
Eq AsyncException | |
Eq ArrayException | |
Eq ExitCode | |
Eq IOErrorType | |
Eq All | |
Eq Any | |
Eq Arity | |
Eq Fixity | |
Eq Associativity | |
Eq TypeRepKey | |
Eq MaskingState | |
Eq IOException | |
Eq ArithException | |
Eq TypeRep | |
Eq TyCon | |
Eq ByteString | |
Eq ByteString | |
Eq Text | |
Eq Root | |
Eq FilePath | |
Eq Text | |
Eq a => Eq [a] | |
Eq a => Eq (Ratio a) | |
Eq a => Eq (Complex a) | |
Eq (TVar a) | |
Eq a => Eq (Dual a) | |
Eq a => Eq (Sum a) | |
Eq a => Eq (Product a) | |
Eq a => Eq (First a) | |
Eq a => Eq (Last a) | |
Eq a => Eq (Down a) | |
Eq a => Eq (Maybe a) | |
Eq a => Eq (Set a) | |
(Hashable a, Eq a) => Eq (HashSet a) | |
Eq a => Eq (Vector a) | |
(Unbox a, Eq a) => Eq (Vector a) | |
(Prim a, Eq a) => Eq (Vector a) | |
(Eq a, Eq b) => Eq (Either a b) | |
(Eq a, Eq b) => Eq (a, b) | |
Eq (m a) => Eq (NonGreedy m a) | |
(Eq a, Eq b) => Eq (:& a b) | |
(Eq k, Eq a) => Eq (Map k a) | |
(Eq k, Eq v) => Eq (Leaf k v) | |
(Eq k, Eq v) => Eq (HashMap k v) | |
Eq a => Eq (Stream Id a) | |
(Eq a, Eq b, Eq c) => Eq (a, b, c) | |
(Eq a, Eq b, Eq c, Eq d) => Eq (a, b, c, d) | |
(Eq a, Eq b, Eq c, Eq d, Eq e) => Eq (a, b, c, d, e) | |
(Eq a, Eq b, Eq c, Eq d, Eq e, Eq f) => Eq (a, b, c, d, e, f) | |
(Eq a, Eq b, Eq c, Eq d, Eq e, Eq f, Eq g) => Eq (a, b, c, d, e, f, g) | |
(Eq a, Eq b, Eq c, Eq d, Eq e, Eq f, Eq g, Eq h) => Eq (a, b, c, d, e, f, g, h) | |
(Eq a, Eq b, Eq c, Eq d, Eq e, Eq f, Eq g, Eq h, Eq i) => Eq (a, b, c, d, e, f, g, h, i) | |
(Eq a, Eq b, Eq c, Eq d, Eq e, Eq f, Eq g, Eq h, Eq i, Eq j) => Eq (a, b, c, d, e, f, g, h, i, j) | |
(Eq a, Eq b, Eq c, Eq d, Eq e, Eq f, Eq g, Eq h, Eq i, Eq j, Eq k) => Eq (a, b, c, d, e, f, g, h, i, j, k) | |
(Eq a, Eq b, Eq c, Eq d, Eq e, Eq f, Eq g, Eq h, Eq i, Eq j, Eq k, Eq l) => Eq (a, b, c, d, e, f, g, h, i, j, k, l) | |
(Eq a, Eq b, Eq c, Eq d, Eq e, Eq f, Eq g, Eq h, Eq i, Eq j, Eq k, Eq l, Eq m) => Eq (a, b, c, d, e, f, g, h, i, j, k, l, m) | |
(Eq a, Eq b, Eq c, Eq d, Eq e, Eq f, Eq g, Eq h, Eq i, Eq j, Eq k, Eq l, Eq m, Eq n) => Eq (a, b, c, d, e, f, g, h, i, j, k, l, m, n) | |
(Eq a, Eq b, Eq c, Eq d, Eq e, Eq f, Eq g, Eq h, Eq i, Eq j, Eq k, Eq l, Eq m, Eq n, Eq o) => Eq (a, b, c, d, e, f, g, h, i, j, k, l, m, n, o) |
class Bounded a where
The Bounded
class is used to name the upper and lower limits of a
type. Ord
is not a superclass of Bounded
since types that are not
totally ordered may also have upper and lower bounds.
The Bounded
class may be derived for any enumeration type;
minBound
is the first constructor listed in the data
declaration
and maxBound
is the last.
Bounded
may also be derived for single-constructor datatypes whose
constituent types are in Bounded
.
class Enum a where
Class Enum
defines operations on sequentially ordered types.
The enumFrom
... methods are used in Haskell's translation of
arithmetic sequences.
Instances of Enum
may be derived for any enumeration type (types
whose constructors have no fields). The nullary constructors are
assumed to be numbered left-to-right by fromEnum
from 0
through n-1
.
See Chapter 10 of the Haskell Report for more details.
For any type that is an instance of class Bounded
as well as Enum
,
the following should hold:
- The calls
andsucc
maxBound
should result in a runtime error.pred
minBound
-
fromEnum
andtoEnum
should give a runtime error if the result value is not representable in the result type. For example,
is an error.toEnum
7 ::Bool
-
enumFrom
andenumFromThen
should be defined with an implicit bound, thus:
enumFrom x = enumFromTo x maxBound enumFromThen x y = enumFromThenTo x y bound where bound | fromEnum y >= fromEnum x = maxBound | otherwise = minBound
succ :: a -> a
the successor of a value. For numeric types, succ
adds 1.
pred :: a -> a
the predecessor of a value. For numeric types, pred
subtracts 1.
Convert from an Int
.
Convert to an Int
.
It is implementation-dependent what fromEnum
returns when
applied to a value that is too large to fit in an Int
.
enumFrom :: a -> [a]
Used in Haskell's translation of [n..]
.
enumFromThen :: a -> a -> [a]
Used in Haskell's translation of [n,n'..]
.
enumFromTo :: a -> a -> [a]
Used in Haskell's translation of [n..m]
.
enumFromThenTo :: a -> a -> a -> [a]
Used in Haskell's translation of [n,n'..m]
.
class Show a
Conversion of values to readable String
s.
Minimal complete definition: showsPrec
or show
.
Derived instances of Show
have the following properties, which
are compatible with derived instances of Read
:
- The result of
show
is a syntactically correct Haskell expression containing only constants, given the fixity declarations in force at the point where the type is declared. It contains only the constructor names defined in the data type, parentheses, and spaces. When labelled constructor fields are used, braces, commas, field names, and equal signs are also used. - If the constructor is defined to be an infix operator, then
showsPrec
will produce infix applications of the constructor. - the representation will be enclosed in parentheses if the
precedence of the top-level constructor in
x
is less thand
(associativity is ignored). Thus, ifd
is0
then the result is never surrounded in parentheses; ifd
is11
it is always surrounded in parentheses, unless it is an atomic expression. - If the constructor is defined using record syntax, then
show
will produce the record-syntax form, with the fields given in the same order as the original declaration.
For example, given the declarations
infixr 5 :^: data Tree a = Leaf a | Tree a :^: Tree a
the derived instance of Show
is equivalent to
instance (Show a) => Show (Tree a) where showsPrec d (Leaf m) = showParen (d > app_prec) $ showString "Leaf " . showsPrec (app_prec+1) m where app_prec = 10 showsPrec d (u :^: v) = showParen (d > up_prec) $ showsPrec (up_prec+1) u . showString " :^: " . showsPrec (up_prec+1) v where up_prec = 5
Note that right-associativity of :^:
is ignored. For example,
-
produces the stringshow
(Leaf 1 :^: Leaf 2 :^: Leaf 3)"Leaf 1 :^: (Leaf 2 :^: Leaf 3)"
.
class Read a
Parsing of String
s, producing values.
Minimal complete definition: readsPrec
(or, for GHC only, readPrec
)
Derived instances of Read
make the following assumptions, which
derived instances of Show
obey:
- If the constructor is defined to be an infix operator, then the
derived
Read
instance will parse only infix applications of the constructor (not the prefix form). - Associativity is not used to reduce the occurrence of parentheses, although precedence may be.
- If the constructor is defined using record syntax, the derived
Read
will parse only the record-syntax form, and furthermore, the fields must be given in the same order as the original declaration. - The derived
Read
instance allows arbitrary Haskell whitespace between tokens of the input string. Extra parentheses are also allowed.
For example, given the declarations
infixr 5 :^: data Tree a = Leaf a | Tree a :^: Tree a
the derived instance of Read
in Haskell 98 is equivalent to
instance (Read a) => Read (Tree a) where readsPrec d r = readParen (d > app_prec) (\r -> [(Leaf m,t) | ("Leaf",s) <- lex r, (m,t) <- readsPrec (app_prec+1) s]) r ++ readParen (d > up_prec) (\r -> [(u:^:v,w) | (u,s) <- readsPrec (up_prec+1) r, (":^:",t) <- lex s, (v,w) <- readsPrec (up_prec+1) t]) r where app_prec = 10 up_prec = 5
Note that right-associativity of :^:
is unused.
The derived instance in GHC is equivalent to
instance (Read a) => Read (Tree a) where readPrec = parens $ (prec app_prec $ do Ident "Leaf" <- lexP m <- step readPrec return (Leaf m)) +++ (prec up_prec $ do u <- step readPrec Symbol ":^:" <- lexP v <- step readPrec return (u :^: v)) where app_prec = 10 up_prec = 5 readListPrec = readListPrecDefault
Read Bool | |
Read Char | |
Read Double | |
Read Float | |
Read Int | |
Read Int8 | |
Read Int16 | |
Read Int32 | |
Read Int64 | |
Read Integer | |
Read Ordering | |
Read Word | |
Read Word8 | |
Read Word16 | |
Read Word32 | |
Read Word64 | |
Read () | |
Read ExitCode | |
Read All | |
Read Any | |
Read Arity | |
Read Fixity | |
Read Associativity | |
Read Lexeme | |
Read ByteString | |
Read ByteString | |
Read Text | |
Read Text | |
Read a => Read [a] | |
(Integral a, Read a) => Read (Ratio a) | |
Read a => Read (Complex a) | |
Read a => Read (Dual a) | |
Read a => Read (Sum a) | |
Read a => Read (Product a) | |
Read a => Read (First a) | |
Read a => Read (Last a) | |
Read a => Read (Maybe a) | |
(Read a, Ord a) => Read (Set a) | |
Read a => Read (Vector a) | |
(Read a, Unbox a) => Read (Vector a) | |
(Read a, Prim a) => Read (Vector a) | |
(Read a, Read b) => Read (Either a b) | |
(Read a, Read b) => Read (a, b) | |
(Ix a, Read a, Read b) => Read (Array a b) | |
(Ord k, Read k, Read e) => Read (Map k e) | |
(Read a, Read b, Read c) => Read (a, b, c) | |
(Read a, Read b, Read c, Read d) => Read (a, b, c, d) | |
(Read a, Read b, Read c, Read d, Read e) => Read (a, b, c, d, e) | |
(Read a, Read b, Read c, Read d, Read e, Read f) => Read (a, b, c, d, e, f) | |
(Read a, Read b, Read c, Read d, Read e, Read f, Read g) => Read (a, b, c, d, e, f, g) | |
(Read a, Read b, Read c, Read d, Read e, Read f, Read g, Read h) => Read (a, b, c, d, e, f, g, h) | |
(Read a, Read b, Read c, Read d, Read e, Read f, Read g, Read h, Read i) => Read (a, b, c, d, e, f, g, h, i) | |
(Read a, Read b, Read c, Read d, Read e, Read f, Read g, Read h, Read i, Read j) => Read (a, b, c, d, e, f, g, h, i, j) | |
(Read a, Read b, Read c, Read d, Read e, Read f, Read g, Read h, Read i, Read j, Read k) => Read (a, b, c, d, e, f, g, h, i, j, k) | |
(Read a, Read b, Read c, Read d, Read e, Read f, Read g, Read h, Read i, Read j, Read k, Read l) => Read (a, b, c, d, e, f, g, h, i, j, k, l) | |
(Read a, Read b, Read c, Read d, Read e, Read f, Read g, Read h, Read i, Read j, Read k, Read l, Read m) => Read (a, b, c, d, e, f, g, h, i, j, k, l, m) | |
(Read a, Read b, Read c, Read d, Read e, Read f, Read g, Read h, Read i, Read j, Read k, Read l, Read m, Read n) => Read (a, b, c, d, e, f, g, h, i, j, k, l, m, n) | |
(Read a, Read b, Read c, Read d, Read e, Read f, Read g, Read h, Read i, Read j, Read k, Read l, Read m, Read n, Read o) => Read (a, b, c, d, e, f, g, h, i, j, k, l, m, n, o) |
class Functor f where
The Functor
class is used for types that can be mapped over.
Instances of Functor
should satisfy the following laws:
fmap id == id fmap (f . g) == fmap f . fmap g
The instances of Functor
for lists, Maybe
and IO
satisfy these laws.
Functor [] | |
Functor IO | |
Functor ZipList | |
Functor Handler | |
Functor STM | |
Functor ReadP | |
Functor Maybe | |
Functor Identity | |
Functor Vector | |
Functor ((->) r) | |
Functor (Either a) | |
Functor ((,) a) | |
Functor (ST s) | |
Functor (Const m) | |
Monad m => Functor (WrappedMonad m) | |
Functor (ST s) | |
Arrow a => Functor (ArrowMonad a) | |
Functor (Map k) | |
Functor m => Functor (MaybeT m) | |
Functor m => Functor (ListT m) | |
Functor m => Functor (IdentityT m) | |
Functor (HashMap k) | |
Arrow a => Functor (WrappedArrow a b) | |
Functor m => Functor (WriterT w m) | |
Functor m => Functor (WriterT w m) | |
Functor m => Functor (StateT s m) | |
Functor m => Functor (StateT s m) | |
Functor m => Functor (ReaderT r m) | |
Functor m => Functor (ErrorT e m) | |
Functor m => Functor (RWST r w s m) | |
Functor m => Functor (RWST r w s m) |
class Monad m where
The Monad
class defines the basic operations over a monad,
a concept from a branch of mathematics known as category theory.
From the perspective of a Haskell programmer, however, it is best to
think of a monad as an abstract datatype of actions.
Haskell's do
expressions provide a convenient syntax for writing
monadic expressions.
Minimal complete definition: >>=
and return
.
Instances of Monad
should satisfy the following laws:
return a >>= k == k a m >>= return == m m >>= (\x -> k x >>= h) == (m >>= k) >>= h
Instances of both Monad
and Functor
should additionally satisfy the law:
fmap f xs == xs >>= return . f
The instances of Monad
for lists, Maybe
and IO
defined in the Prelude satisfy these laws.
(>>=) :: m a -> (a -> m b) -> m b
Sequentially compose two actions, passing any value produced by the first as an argument to the second.
(>>) :: m a -> m b -> m b
Sequentially compose two actions, discarding any value produced by the first, like sequencing operators (such as the semicolon) in imperative languages.
return :: a -> m a
Inject a value into the monadic type.
Fail with a message. This operation is not part of the
mathematical definition of a monad, but is invoked on pattern-match
failure in a do
expression.
Monad [] | |
Monad IO | |
Monad P | |
Monad STM | |
Monad ReadP | |
Monad Maybe | |
Monad Identity | |
Monad Vector | |
Monad ((->) r) | |
Monad (Either e) | |
Monad (ST s) | |
Monad (ST s) | |
ArrowApply a => Monad (ArrowMonad a) | |
Monad m => Monad (MaybeT m) | |
Monad m => Monad (ListT m) | |
Monad m => Monad (IdentityT m) | |
(Monoid w, Monad m) => Monad (WriterT w m) | |
(Monoid w, Monad m) => Monad (WriterT w m) | |
Monad m => Monad (StateT s m) | |
Monad m => Monad (StateT s m) | |
Monad m => Monad (ReaderT r m) | |
(Monad m, Error e) => Monad (ErrorT e m) | |
(Monoid w, Monad m) => Monad (RWST r w s m) | |
(Monoid w, Monad m) => Monad (RWST r w s m) |
class IsString a where
Class for string-like datastructures; used by the overloaded string extension (-foverloaded-strings in GHC).
fromString :: String -> a
Numeric type classes
class Num a where
Basic numeric class.
Minimal complete definition: all except negate
or (-)
(+) :: a -> a -> a
(*) :: a -> a -> a
(-) :: a -> a -> a
negate :: a -> a
Unary negation.
abs :: a -> a
Absolute value.
signum :: a -> a
Sign of a number.
The functions abs
and signum
should satisfy the law:
abs x * signum x == x
For real numbers, the signum
is either -1
(negative), 0
(zero)
or 1
(positive).
fromInteger :: Integer -> a
Conversion from an Integer
.
An integer literal represents the application of the function
fromInteger
to the appropriate value of type Integer
,
so such literals have type (
.
Num
a) => a
class (Num a, Ord a) => Real a where
toRational :: a -> Rational
the rational equivalent of its real argument with full precision
class (Real a, Enum a) => Integral a where
quot :: a -> a -> a
integer division truncated toward zero
rem :: a -> a -> a
integer remainder, satisfying
(x `quot` y)*y + (x `rem` y) == x
div :: a -> a -> a
integer division truncated toward negative infinity
mod :: a -> a -> a
integer modulus, satisfying
(x `div` y)*y + (x `mod` y) == x
quotRem :: a -> a -> (a, a)
divMod :: a -> a -> (a, a)
conversion to Integer
class Num a => Fractional a where
Fractional numbers, supporting real division.
Minimal complete definition: fromRational
and (recip
or (
)
/
)
(/) :: a -> a -> a
fractional division
recip :: a -> a
reciprocal fraction
fromRational :: Rational -> a
Conversion from a Rational
(that is
).
A floating literal stands for an application of Ratio
Integer
fromRational
to a value of type Rational
, so such literals have type
(
.
Fractional
a) => a
Fractional Double | |
Fractional Float | |
(Num (Ratio a), Integral a) => Fractional (Ratio a) | |
(Num (Complex a), RealFloat a) => Fractional (Complex a) |
class Fractional a => Floating a where
Trigonometric and hyperbolic functions and related functions.
Minimal complete definition:
pi
, exp
, log
, sin
, cos
, sinh
, cosh
,
asin
, acos
, atan
, asinh
, acosh
and atanh
class (Real a, Fractional a) => RealFrac a where
Extracting components of fractions.
Minimal complete definition: properFraction
properFraction :: Integral b => a -> (b, a)
The function properFraction
takes a real fractional number x
and returns a pair (n,f)
such that x = n+f
, and:
-
n
is an integral number with the same sign asx
; and -
f
is a fraction with the same type and sign asx
, and with absolute value less than1
.
The default definitions of the ceiling
, floor
, truncate
and round
functions are in terms of properFraction
.
truncate :: Integral b => a -> b
returns the integer nearest truncate
xx
between zero and x
returns the nearest integer to round
xx
;
the even integer if x
is equidistant between two integers
ceiling :: Integral b => a -> b
returns the least integer not less than ceiling
xx
returns the greatest integer not greater than floor
xx
class (RealFrac a, Floating a) => RealFloat a where
Efficient, machine-independent access to the components of a floating-point number.
Minimal complete definition:
all except exponent
, significand
, scaleFloat
and atan2
floatRadix :: a -> Integer
a constant function, returning the radix of the representation
(often 2
)
floatDigits :: a -> Int
a constant function, returning the number of digits of
floatRadix
in the significand
floatRange :: a -> (Int, Int)
a constant function, returning the lowest and highest values the exponent may assume
decodeFloat :: a -> (Integer, Int)
The function decodeFloat
applied to a real floating-point
number returns the significand expressed as an Integer
and an
appropriately scaled exponent (an Int
). If
yields decodeFloat
x(m,n)
, then x
is equal in value to m*b^^n
, where b
is the floating-point radix, and furthermore, either m
and n
are both zero or else b^(d-1) <=
, where abs
m < b^dd
is
the value of
.
In particular, floatDigits
x
. If the type
contains a negative zero, also decodeFloat
0 = (0,0)
.
The result of decodeFloat
(-0.0) = (0,0)
is unspecified if either of
decodeFloat
x
or isNaN
x
is isInfinite
xTrue
.
encodeFloat :: Integer -> Int -> a
encodeFloat
performs the inverse of decodeFloat
in the
sense that for finite x
with the exception of -0.0
,
.
uncurry
encodeFloat
(decodeFloat
x) = x
is one of the two closest representable
floating-point numbers to encodeFloat
m nm*b^^n
(or ±Infinity
if overflow
occurs); usually the closer, but if m
contains too many bits,
the result may be rounded in the wrong direction.
exponent
corresponds to the second component of decodeFloat
.
and for finite nonzero exponent
0 = 0x
,
.
If exponent
x = snd (decodeFloat
x) + floatDigits
xx
is a finite floating-point number, it is equal in value to
, where significand
x * b ^^ exponent
xb
is the
floating-point radix.
The behaviour is unspecified on infinite or NaN
values.
significand :: a -> a
The first component of decodeFloat
, scaled to lie in the open
interval (-1
,1
), either 0.0
or of absolute value >= 1/b
,
where b
is the floating-point radix.
The behaviour is unspecified on infinite or NaN
values.
scaleFloat :: Int -> a -> a
multiplies a floating-point number by an integer power of the radix
True
if the argument is an IEEE "not-a-number" (NaN) value
isInfinite :: a -> Bool
True
if the argument is an IEEE infinity or negative infinity
isDenormalized :: a -> Bool
True
if the argument is too small to be represented in
normalized format
isNegativeZero :: a -> Bool
True
if the argument is an IEEE negative zero
True
if the argument is an IEEE floating point number
atan2 :: a -> a -> a
a version of arctangent taking two real floating-point arguments.
For real floating x
and y
,
computes the angle
(from the positive x-axis) of the vector from the origin to the
point atan2
y x(x,y)
.
returns a value in the range [atan2
y x-pi
,
pi
]. It follows the Common Lisp semantics for the origin when
signed zeroes are supported.
, with atan2
y 1y
in a type
that is RealFloat
, should return the same value as
.
A default definition of atan
yatan2
is provided, but implementors
can provide a more accurate implementation.
Data types
data Maybe a
The Maybe
type encapsulates an optional value. A value of type
either contains a value of type Maybe
aa
(represented as
),
or it is empty (represented as Just
aNothing
). Using Maybe
is a good way to
deal with errors or exceptional cases without resorting to drastic
measures such as error
.
The Maybe
type is also a monad. It is a simple kind of error
monad, where all errors are represented by Nothing
. A richer
error monad can be built using the Either
type.
Monad Maybe | |
Functor Maybe | |
Typeable1 Maybe | |
MonadPlus Maybe | |
Applicative Maybe | |
Generic1 Maybe | |
Alternative Maybe | |
MonadBase Maybe Maybe | |
MonadBaseControl Maybe Maybe | |
Eq a => Eq (Maybe a) | |
(Eq (Maybe a), Ord a) => Ord (Maybe a) | |
Read a => Read (Maybe a) | |
Show a => Show (Maybe a) | |
Generic (Maybe a) | |
Arguable a => Argument (Maybe a) | use Maybe when it should be parsed from one or zero (greedily) |
Monoid a => Monoid (Maybe a) | Lift a semigroup into |
Hashable a => Hashable (Maybe a) |
data Ordering
data Bool
data Char
The character type Char
is an enumeration whose values represent
Unicode (or equivalently ISO/IEC 10646) characters (see
http://www.unicode.org/ for details). This set extends the ISO 8859-1
(Latin-1) character set (the first 256 characters), which is itself an extension
of the ASCII character set (the first 128 characters). A character literal in
Haskell has type Char
.
To convert a Char
to or from the corresponding Int
value defined
by Unicode, use toEnum
and fromEnum
from the
Enum
class respectively (or equivalently ord
and chr
).
Bounded Char | |
Enum Char | |
Eq Char | |
Ord Char | |
Read Char | |
Show Char | |
Typeable Char | |
Generic Char | |
Arguable Char | char is a special case, so that we don't force the user to single-quote their input |
Arguable String | string is a special case, so that we don't force the user to double-quote their input |
Argument String | make sure strings are handled as a separate type, not a list of chars |
Hashable Char | |
ErrorList Char | |
Unbox Char | |
Vector Vector Char | |
MVector MVector Char | |
IsString [Char] |
data IO a
A value of type
is a computation which, when performed,
does some I/O before returning a value of type IO
aa
.
There is really only one way to "perform" an I/O action: bind it to
Main.main
in your program. When your program is run, the I/O will
be performed. It isn't possible to perform I/O from an arbitrary
function, unless that function is itself in the IO
monad and called
at some point, directly or indirectly, from Main.main
.
IO
is a monad, so IO
actions can be combined using either the do-notation
or the >>
and >>=
operations from the Monad
class.
data Either a b
The Either
type represents values with two possibilities: a value of
type
is either Either
a b
or Left
a
.
Right
b
The Either
type is sometimes used to represent a value which is
either correct or an error; by convention, the Left
constructor is
used to hold an error value and the Right
constructor is used to
hold a correct value (mnemonic: "right" also means "correct").
Typeable2 Either | |
Monad (Either e) | |
Functor (Either a) | |
(Monad (Either e), Error e) => MonadPlus (Either e) | |
Functor (Either e) => Applicative (Either e) | |
Generic1 (Either a) | |
(Applicative (Either e), Error e) => Alternative (Either e) | |
(Applicative (Either e), Applicative (Either e), Monad (Either e), Monad (Either e)) => MonadBase (Either e) (Either e) | |
MonadBase (Either e) (Either e) => MonadBaseControl (Either e) (Either e) | |
(Eq a, Eq b) => Eq (Either a b) | |
(Eq (Either a b), Ord a, Ord b) => Ord (Either a b) | |
(Read a, Read b) => Read (Either a b) | |
(Show a, Show b) => Show (Either a b) | |
Generic (Either a b) | |
(Hashable a, Hashable b) => Hashable (Either a b) |
Re-exports
Packed reps
data ByteString
A space-efficient representation of a Word8 vector, supporting many
efficient operations. A ByteString
contains 8-bit characters only.
Instances of Eq, Ord, Read, Show, Data, Typeable
type LByteString = ByteStringSource
data Text
A space efficient, packed, unboxed Unicode text type.
Containers
data Map k a
A Map from keys k
to values a
.
Typeable2 Map | |
Functor (Map k) | |
Foldable (Map k) | |
(Functor (Map k), Foldable (Map k)) => Traversable (Map k) | |
(Eq k, Eq a) => Eq (Map k a) | |
(Typeable (Map k a), Data k, Data a, Ord k) => Data (Map k a) | |
(Eq (Map k v), Ord k, Ord v) => Ord (Map k v) | |
(Ord k, Read k, Read e) => Read (Map k e) | |
(Show k, Show a) => Show (Map k a) | |
Ord k => Monoid (Map k v) | |
(NFData k, NFData a) => NFData (Map k a) |
data HashMap k v
A map from keys to values. A map cannot contain duplicate keys; each key can map to at most one value.
Typeable2 HashMap | |
Functor (HashMap k) | |
Foldable (HashMap k) | |
(Functor (HashMap k), Foldable (HashMap k)) => Traversable (HashMap k) | |
(Eq k, Eq v) => Eq (HashMap k v) | |
(Typeable (HashMap k v), Data k, Data v, Eq k, Hashable k) => Data (HashMap k v) | |
(Show k, Show v) => Show (HashMap k v) | |
(Eq k, Hashable k) => Monoid (HashMap k v) | |
(NFData k, NFData v) => NFData (HashMap k v) |
data Set a
A set of values a
.
data HashSet a
A set of values. A set cannot contain duplicate values.
data Vector a
Boxed vectors, supporting efficient slicing.
Monad Vector | |
Functor Vector | |
Typeable1 Vector | |
MonadPlus Vector | |
Applicative Vector | |
Foldable Vector | |
Traversable Vector | |
Alternative Vector | |
MVector (Mutable Vector) a => Vector Vector a | |
Eq a => Eq (Vector a) | |
(Typeable (Vector a), Data a) => Data (Vector a) | |
(Eq (Vector a), Ord a) => Ord (Vector a) | |
Read a => Read (Vector a) | |
Show a => Show (Vector a) | |
Monoid (Vector a) | |
NFData a => NFData (Vector a) |
class (Vector Vector a, MVector MVector a) => Unbox a
Unbox Bool | |
Unbox Char | |
Unbox Double | |
Unbox Float | |
Unbox Int | |
Unbox Int8 | |
Unbox Int16 | |
Unbox Int32 | |
Unbox Int64 | |
Unbox Word | |
Unbox Word8 | |
Unbox Word16 | |
Unbox Word32 | |
Unbox Word64 | |
Unbox () | |
(Vector Vector (Complex a), MVector MVector (Complex a), RealFloat a, Unbox a) => Unbox (Complex a) | |
(Vector Vector (a, b), MVector MVector (a, b), Unbox a, Unbox b) => Unbox (a, b) | |
(Vector Vector (a, b, c), MVector MVector (a, b, c), Unbox a, Unbox b, Unbox c) => Unbox (a, b, c) | |
(Vector Vector (a, b, c, d), MVector MVector (a, b, c, d), Unbox a, Unbox b, Unbox c, Unbox d) => Unbox (a, b, c, d) | |
(Vector Vector (a, b, c, d, e), MVector MVector (a, b, c, d, e), Unbox a, Unbox b, Unbox c, Unbox d, Unbox e) => Unbox (a, b, c, d, e) | |
(Vector Vector (a, b, c, d, e, f), MVector MVector (a, b, c, d, e, f), Unbox a, Unbox b, Unbox c, Unbox d, Unbox e, Unbox f) => Unbox (a, b, c, d, e, f) |
class Hashable a
The class of types that can be converted to a hash value.
Numbers
data Word
data Word8
8-bit unsigned integer type
data Word32
32-bit unsigned integer type
data Word64
64-bit unsigned integer type
data Int
data Int32
32-bit signed integer type
data Int64
64-bit signed integer type
data Integer
Arbitrary-precision integers.
data Float
Single-precision floating point numbers. It is desirable that this type be at least equal in range and precision to the IEEE single-precision type.
data Double
Double-precision floating point numbers. It is desirable that this type be at least equal in range and precision to the IEEE double-precision type.
Numeric functions
(^^) :: (Fractional a, Integral b) => a -> b -> a
raise a number to an integral power
fromIntegral :: (Integral a, Num b) => a -> b
general coercion from integral types
realToFrac :: (Real a, Fractional b) => a -> b
general coercion to fractional types
Monoids
class Monoid a where
The class of monoids (types with an associative binary operation that has an identity). Instances should satisfy the following laws:
mappend mempty x = x
mappend x mempty = x
mappend x (mappend y z) = mappend (mappend x y) z
mconcat =
foldr
mappend mempty
The method names refer to the monoid of lists under concatenation, but there are many other instances.
Minimal complete definition: mempty
and mappend
.
Some types can be viewed as a monoid in more than one way,
e.g. both addition and multiplication on numbers.
In such cases we often define newtype
s and make those instances
of Monoid
, e.g. Sum
and Product
.
mempty :: a
Identity of mappend
mappend :: a -> a -> a
An associative operation
mconcat :: [a] -> a
Fold a list using the monoid.
For most types, the default definition for mconcat
will be
used, but the function is included in the class definition so
that an optimized version can be provided for specific types.
Monoid Ordering | |
Monoid () | |
Monoid All | |
Monoid Any | |
Monoid ByteString | |
Monoid ByteString | |
Monoid Text | |
Monoid FilePath | |
Monoid Text | |
Monoid [a] | |
Monoid a => Monoid (Dual a) | |
Monoid (Endo a) | |
Num a => Monoid (Sum a) | |
Num a => Monoid (Product a) | |
Monoid (First a) | |
Monoid (Last a) | |
Monoid a => Monoid (Maybe a) | Lift a semigroup into |
Ord a => Monoid (Set a) | |
(Hashable a, Eq a) => Monoid (HashSet a) | |
Monoid (Vector a) | |
Unbox a => Monoid (Vector a) | |
Prim a => Monoid (Vector a) | |
Monoid b => Monoid (a -> b) | |
(Monoid a, Monoid b) => Monoid (a, b) | |
Ord k => Monoid (Map k v) | |
(Eq k, Hashable k) => Monoid (HashMap k v) | |
(Monoid a, Monoid b, Monoid c) => Monoid (a, b, c) | |
(Monoid a, Monoid b, Monoid c, Monoid d) => Monoid (a, b, c, d) | |
(Monoid a, Monoid b, Monoid c, Monoid d, Monoid e) => Monoid (a, b, c, d, e) |
Arrow
first :: Arrow a => forall b c d. a b c -> a (b, d) (c, d)
Send the first component of the input through the argument arrow, and copy the rest unchanged to the output.
second :: Arrow a => forall b c d. a b c -> a (d, b) (d, c)
A mirror image of first
.
The default definition may be overridden with a more efficient version if desired.
(***) :: Arrow a => forall b c b' c'. a b c -> a b' c' -> a (b, b') (c, c')
Split the input between the two argument arrows and combine their output. Note that this is in general not a functor.
The default definition may be overridden with a more efficient version if desired.
(&&&) :: Arrow a => forall b c c'. a b c -> a b c' -> a b (c, c')
Fanout: send the input to both argument arrows and combine their output.
The default definition may be overridden with a more efficient version if desired.
Maybe
listToMaybe :: [a] -> Maybe a
The listToMaybe
function returns Nothing
on an empty list
or
where Just
aa
is the first element of the list.
maybeToList :: Maybe a -> [a]
The maybeToList
function returns an empty list when given
Nothing
or a singleton list when not given Nothing
.
Either
partitionEithers :: [Either a b] -> ([a], [b])
Ord
on :: (b -> b -> c) -> (a -> b) -> a -> a -> c
comparing :: Ord a => (b -> a) -> b -> b -> Ordering
comparing p x y = compare (p x) (p y)
Useful combinator for use in conjunction with the xxxBy
family
of functions from Data.List, for example:
... sortBy (comparing fst) ...
Applicative
class Functor f => Applicative f where
A functor with application, providing operations to
A minimal complete definition must include implementations of these functions satisfying the following laws:
- identity
-
pure
id
<*>
v = v - composition
-
pure
(.)<*>
u<*>
v<*>
w = u<*>
(v<*>
w) - homomorphism
-
pure
f<*>
pure
x =pure
(f x) - interchange
-
u
<*>
pure
y =pure
($
y)<*>
u
The other methods have the following default definitions, which may be overridden with equivalent specialized implementations:
u*>
v =pure
(const
id
)<*>
u<*>
v u<*
v =pure
const
<*>
u<*>
v
As a consequence of these laws, the Functor
instance for f
will satisfy
fmap
f x =pure
f<*>
x
If f
is also a Monad
, it should satisfy
and
pure
= return
(
(which implies that <*>
) = ap
pure
and <*>
satisfy the
applicative functor laws).
pure :: a -> f a
Lift a value.
(<*>) :: f (a -> b) -> f a -> f b
Sequential application.
(*>) :: f a -> f b -> f b
Sequence actions, discarding the value of the first argument.
(<*) :: f a -> f b -> f a
Sequence actions, discarding the value of the second argument.
Monad
(>=>) :: Monad m => (a -> m b) -> (b -> m c) -> a -> m c
Left-to-right Kleisli composition of monads.
Transformers
lift :: MonadTrans t => forall m a. Monad m => m a -> t m a
Lift a computation from the argument monad to the constructed monad.
class Monad m => MonadIO m where
Monads in which IO
computations may be embedded.
Any monad built by applying a sequence of monad transformers to the
IO
monad will be an instance of this class.
Instances should satisfy the following laws, which state that liftIO
is a transformer of monads:
MonadIO IO | |
(Monad (MaybeT m), MonadIO m) => MonadIO (MaybeT m) | |
(Monad (ListT m), MonadIO m) => MonadIO (ListT m) | |
(Monad (IdentityT m), MonadIO m) => MonadIO (IdentityT m) | |
(Monad (WriterT w m), Monoid w, MonadIO m) => MonadIO (WriterT w m) | |
(Monad (WriterT w m), Monoid w, MonadIO m) => MonadIO (WriterT w m) | |
(Monad (StateT s m), MonadIO m) => MonadIO (StateT s m) | |
(Monad (StateT s m), MonadIO m) => MonadIO (StateT s m) | |
(Monad (ReaderT r m), MonadIO m) => MonadIO (ReaderT r m) | |
(Monad (ErrorT e m), Error e, MonadIO m) => MonadIO (ErrorT e m) | |
(Monad (RWST r w s m), Monoid w, MonadIO m) => MonadIO (RWST r w s m) | |
(Monad (RWST r w s m), Monoid w, MonadIO m) => MonadIO (RWST r w s m) |
Exceptions
class (Typeable e, Show e) => Exception e where
Any type that you wish to throw or catch as an exception must be an
instance of the Exception
class. The simplest case is a new exception
type directly below the root:
data MyException = ThisException | ThatException deriving (Show, Typeable) instance Exception MyException
The default method definitions in the Exception
class do what we need
in this case. You can now throw and catch ThisException
and
ThatException
as exceptions:
*Main> throw ThisException `catch` \e -> putStrLn ("Caught " ++ show (e :: MyException)) Caught ThisException
In more complicated examples, you may wish to define a whole hierarchy of exceptions:
--------------------------------------------------------------------- -- Make the root exception type for all the exceptions in a compiler data SomeCompilerException = forall e . Exception e => SomeCompilerException e deriving Typeable instance Show SomeCompilerException where show (SomeCompilerException e) = show e instance Exception SomeCompilerException compilerExceptionToException :: Exception e => e -> SomeException compilerExceptionToException = toException . SomeCompilerException compilerExceptionFromException :: Exception e => SomeException -> Maybe e compilerExceptionFromException x = do SomeCompilerException a <- fromException x cast a --------------------------------------------------------------------- -- Make a subhierarchy for exceptions in the frontend of the compiler data SomeFrontendException = forall e . Exception e => SomeFrontendException e deriving Typeable instance Show SomeFrontendException where show (SomeFrontendException e) = show e instance Exception SomeFrontendException where toException = compilerExceptionToException fromException = compilerExceptionFromException frontendExceptionToException :: Exception e => e -> SomeException frontendExceptionToException = toException . SomeFrontendException frontendExceptionFromException :: Exception e => SomeException -> Maybe e frontendExceptionFromException x = do SomeFrontendException a <- fromException x cast a --------------------------------------------------------------------- -- Make an exception type for a particular frontend compiler exception data MismatchedParentheses = MismatchedParentheses deriving (Typeable, Show) instance Exception MismatchedParentheses where toException = frontendExceptionToException fromException = frontendExceptionFromException
We can now catch a MismatchedParentheses
exception as
MismatchedParentheses
, SomeFrontendException
or
SomeCompilerException
, but not other types, e.g. IOException
:
*Main> throw MismatchedParenthesescatch
e -> putStrLn ("Caught " ++ show (e :: MismatchedParentheses)) Caught MismatchedParentheses *Main> throw MismatchedParenthesescatch
e -> putStrLn ("Caught " ++ show (e :: SomeFrontendException)) Caught MismatchedParentheses *Main> throw MismatchedParenthesescatch
e -> putStrLn ("Caught " ++ show (e :: SomeCompilerException)) Caught MismatchedParentheses *Main> throw MismatchedParenthesescatch
e -> putStrLn ("Caught " ++ show (e :: IOException)) *** Exception: MismatchedParentheses
toException :: e -> SomeException
fromException :: SomeException -> Maybe e
class Typeable a where
The class Typeable
allows a concrete representation of a type to
be calculated.
data SomeException
The SomeException
type is the root of the exception type hierarchy.
When an exception of type e
is thrown, behind the scenes it is
encapsulated in a SomeException
.
data IOException
Exceptions that occur in the IO
monad.
An IOException
records a more specific error type, a descriptive
string and maybe the handle that was used when the error was
flagged.
try :: (MonadBaseControl IO m, Exception e) => m a -> m (Either e a)
Generalized version of try
.
Note, when the given computation throws an exception any monadic
side effects in m
will be discarded.
:: (MonadBaseControl IO m, Exception e) | |
=> m a | The computation to run |
-> (e -> m a) | Handler to invoke if an exception is raised |
-> m a |
Generalized version of catch
.
Note, when the given computation throws an exception any monadic
side effects in m
will be discarded.
:: MonadBaseControl IO m | |
=> m a | computation to run first ("acquire resource") |
-> (a -> m b) | computation to run last ("release resource") |
-> (a -> m c) | computation to run in-between |
-> m c |
Generalized version of bracket
.
Note:
- When the "acquire" or "release" computations throw exceptions
any monadic side effects in
m
will be discarded. - When the "in-between" computation throws an exception any
monadic side effects in
m
produced by that computation will be discarded but the side effects of the "acquire" or "release" computations will be retained. - Also, any monadic side effects in
m
of the "release" computation will be discarded; it is run only for its side effects inIO
.
Note that when your acquire
and release
computations are of type IO
it will be more efficient to write:
liftBaseOp
(bracket
acquire release)
onException :: MonadBaseControl IO m => m a -> m b -> m a
Generalized version of onException
.
Note, any monadic side effects in m
of the "afterward"
computation will be discarded.
:: MonadBaseControl IO m | |
=> m a | computation to run first |
-> m b | computation to run afterward (even if an exception was raised) |
-> m a |
Generalized version of finally
.
Note, any monadic side effects in m
of the "afterward"
computation will be discarded.
Files
data FilePath
(<.>) :: FilePath -> Text -> FilePath
An alias for addExtension
.
hasExtension :: FilePath -> Text -> Bool
Get whether a FilePath
’s last extension is the predicate.
basename :: FilePath -> FilePath
Retrieve a FilePath
’s basename component.
basename "foo/bar.txt" == "bar"
filename :: FilePath -> FilePath
Retrieve a FilePath
’s filename component.
filename "foo/bar.txt" == "bar.txt"
directory :: FilePath -> FilePath
Retrieves the FilePath
’s directory. If the path is already a
directory, it is returned unchanged.
The print
function outputs a value of any printable type to the
standard output device.
Printable types are those that are instances of class Show
; print
converts values to strings for output using the show
operation and
adds a newline.
For example, a program to print the first 20 integers and their powers of 2 could be written as:
main = print ([(n, 2^n) | n <- [0..19]])
Command line args
readArgs :: ArgumentTuple a => IO a
parse the desired argument tuple from the command line or print a simple usage statment and quit