| Safe Haskell | None |
|---|---|
| Language | Haskell98 |
Prelude
- data Char :: *
- type String = [Char]
- data Double :: *
- data Int :: *
- data Integer :: *
- data Bool :: *
- class Read a
- class Show a
- class Eq a where
- (==) :: Eq a => a -> a -> Bool
- (/=) :: Eq a => a -> a -> Bool
- data Maybe a :: * -> *
- maybe :: t -> (t1 -> t) -> Maybe t1 -> t
- (>>=) :: Ptr (Fay a) -> Ptr (a -> Fay b) -> Ptr (Fay b)
- (>>) :: Ptr (Fay a) -> Ptr (Fay b) -> Ptr (Fay b)
- return :: a -> Fay a
- fail :: String -> Fay a
- when :: Bool -> Fay () -> Fay ()
- unless :: Bool -> Fay () -> Fay ()
- forM :: [a] -> (a -> Fay b) -> Fay [b]
- forM_ :: [a] -> (a -> Fay b) -> Fay ()
- mapM :: (a -> Fay b) -> [a] -> Fay [b]
- mapM_ :: (a -> Fay b) -> [a] -> Fay ()
- (=<<) :: (a -> Fay b) -> Fay a -> Fay b
- sequence :: [Fay a] -> Fay [a]
- sequence_ :: [Fay a] -> Fay ()
- void :: Fay a -> Fay ()
- (>=>) :: (a -> Fay b) -> (b -> Fay c) -> a -> Fay c
- (<=<) :: (b -> Fay c) -> (a -> Fay b) -> a -> Fay c
- (*) :: Num a => a -> a -> a
- (+) :: Num a => a -> a -> a
- (-) :: Num a => a -> a -> a
- class Eq a => Ord a where
- data Ordering :: *
- (<) :: Ord a => a -> a -> Bool
- (<=) :: Ord a => a -> a -> Bool
- (>) :: Ord a => a -> a -> Bool
- (>=) :: Ord a => a -> a -> Bool
- compare :: Ord a => a -> a -> Ordering
- succ :: Num a => a -> a
- pred :: Num a => a -> a
- enumFrom :: Num a => a -> [a]
- enumFromTo :: (Ord t, Num t) => t -> t -> [t]
- enumFromBy :: Num t => t -> t -> [t]
- enumFromThen :: Num t => t -> t -> [t]
- enumFromByTo :: (Ord t, Num t) => t -> t -> t -> [t]
- enumFromThenTo :: (Ord t, Num t) => t -> t -> t -> [t]
- (/) :: Fractional a => a -> a -> a
- fromIntegral :: (Num a, Num b) => Ptr a -> Ptr b
- fromInteger :: Num a => Ptr Integer -> Ptr a
- (&&) :: Bool -> Bool -> Bool
- (||) :: Bool -> Bool -> Bool
- not :: Bool -> Bool
- otherwise :: Bool
- show :: Automatic a -> String
- error :: String -> a
- undefined :: a
- data Either a b :: * -> * -> *
- either :: (a -> c) -> (b -> c) -> Either a b -> c
- until :: (a -> Bool) -> (a -> a) -> a -> a
- ($!) :: (a -> b) -> a -> b
- seq :: a -> b -> b
- const :: a -> b -> a
- id :: a -> a
- (.) :: (t1 -> t) -> (t2 -> t1) -> t2 -> t
- ($) :: (t1 -> t) -> t1 -> t
- flip :: (t1 -> t2 -> t) -> t2 -> t1 -> t
- curry :: ((a, b) -> c) -> a -> b -> c
- uncurry :: (a -> b -> c) -> (a, b) -> c
- snd :: (t, t1) -> t1
- fst :: (t, t1) -> t
- div :: Int -> Int -> Int
- mod :: Int -> Int -> Int
- divMod :: Int -> Int -> (Int, Int)
- min :: Num a => a -> a -> a
- max :: Num a => a -> a -> a
- recip :: Double -> Double
- negate :: Num a => a -> a
- abs :: (Num a, Ord a) => a -> a
- signum :: (Num a, Ord a) => a -> a
- pi :: Double
- exp :: Double -> Double
- sqrt :: Double -> Double
- log :: Double -> Double
- (**) :: Double -> Double -> Double
- (^^) :: Double -> Int -> Double
- unsafePow :: (Num a, Num b) => a -> b -> a
- (^) :: Num a => a -> Int -> a
- logBase :: Double -> Double -> Double
- sin :: Double -> Double
- tan :: Double -> Double
- cos :: Double -> Double
- asin :: Double -> Double
- atan :: Double -> Double
- acos :: Double -> Double
- sinh :: Double -> Double
- tanh :: Double -> Double
- cosh :: Double -> Double
- asinh :: Double -> Double
- atanh :: Double -> Double
- acosh :: Double -> Double
- properFraction :: Double -> (Int, Double)
- truncate :: Double -> Int
- round :: Double -> Int
- ceiling :: Double -> Int
- floor :: Double -> Int
- subtract :: Num a => a -> a -> a
- even :: Int -> Bool
- odd :: Int -> Bool
- gcd :: Int -> Int -> Int
- quot :: Int -> Int -> Int
- quot' :: Int -> Int -> Int
- quotRem :: Int -> Int -> (Int, Int)
- rem :: Int -> Int -> Int
- rem' :: Int -> Int -> Int
- lcm :: Int -> Int -> Int
- find :: (a -> Bool) -> [a] -> Maybe a
- filter :: (a -> Bool) -> [a] -> [a]
- null :: [t] -> Bool
- map :: (a -> b) -> [a] -> [b]
- nub :: Eq a => [a] -> [a]
- nub' :: Eq a => [a] -> [a] -> [a]
- elem :: Eq a => a -> [a] -> Bool
- notElem :: Eq a => a -> [a] -> Bool
- sort :: Ord a => [a] -> [a]
- sortBy :: (t -> t -> Ordering) -> [t] -> [t]
- insertBy :: (a -> a -> Ordering) -> a -> [a] -> [a]
- conc :: [a] -> [a] -> [a]
- concat :: [[a]] -> [a]
- concatMap :: (a -> [b]) -> [a] -> [b]
- foldr :: (t -> t1 -> t1) -> t1 -> [t] -> t1
- foldr1 :: (a -> a -> a) -> [a] -> a
- foldl :: (t1 -> t -> t1) -> t1 -> [t] -> t1
- foldl1 :: (a -> a -> a) -> [a] -> a
- (++) :: [a] -> [a] -> [a]
- (!!) :: [a] -> Int -> a
- head :: [a] -> a
- tail :: [a] -> [a]
- init :: [a] -> [a]
- last :: [a] -> a
- iterate :: (a -> a) -> a -> [a]
- repeat :: a -> [a]
- replicate :: Int -> a -> [a]
- cycle :: [a] -> [a]
- take :: Int -> [a] -> [a]
- drop :: Int -> [a] -> [a]
- splitAt :: Int -> [a] -> ([a], [a])
- takeWhile :: (a -> Bool) -> [a] -> [a]
- dropWhile :: (a -> Bool) -> [a] -> [a]
- span :: (a -> Bool) -> [a] -> ([a], [a])
- break :: (a -> Bool) -> [a] -> ([a], [a])
- zipWith :: (a -> b -> c) -> [a] -> [b] -> [c]
- zipWith3 :: (a -> b -> c -> d) -> [a] -> [b] -> [c] -> [d]
- zip :: [a] -> [b] -> [(a, b)]
- zip3 :: [a] -> [b] -> [c] -> [(a, b, c)]
- unzip :: [(a, b)] -> ([a], [b])
- unzip3 :: [(a, b, c)] -> ([a], [b], [c])
- lines :: String -> [String]
- unlines :: [String] -> String
- words :: String -> [String]
- unwords :: [String] -> String
- and :: [Bool] -> Bool
- or :: [Bool] -> Bool
- any :: (a -> Bool) -> [a] -> Bool
- all :: (a -> Bool) -> [a] -> Bool
- intersperse :: a -> [a] -> [a]
- prependToAll :: a -> [a] -> [a]
- intercalate :: [a] -> [[a]] -> [a]
- maximum :: Num a => [a] -> a
- minimum :: Num a => [a] -> a
- product :: Num a => [a] -> a
- sum :: Num a => [a] -> a
- scanl :: (a -> b -> a) -> a -> [b] -> [a]
- scanl1 :: (a -> a -> a) -> [a] -> [a]
- scanr :: (a -> b -> b) -> b -> [a] -> [b]
- scanr1 :: (a -> a -> a) -> [a] -> [a]
- lookup :: Eq a1 => a1 -> [(a1, a)] -> Maybe a
- length :: [a] -> Int
- length' :: Int -> [a] -> Int
- reverse :: [a] -> [a]
- print :: Automatic a -> Fay ()
- putStrLn :: String -> Fay ()
- ifThenElse :: Bool -> t -> t -> t
- data Fay a :: * -> *
Documentation
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).
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.
Parsing of Strings, producing values.
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
Readinstance 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
Readwill parse only the record-syntax form, and furthermore, the fields must be given in the same order as the original declaration. - The derived
Readinstance 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 2010 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 = 5Note 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 = readListPrecDefaultWhy do both readsPrec and readPrec exist, and why does GHC opt to
implement readPrec in derived Read instances instead of readsPrec?
The reason is that readsPrec is based on the ReadS type, and although
ReadS is mentioned in the Haskell 2010 Report, it is not a very efficient
parser data structure.
readPrec, on the other hand, is based on a much more efficient ReadPrec
datatype (a.k.a "new-style parsers"), but its definition relies on the use
of the RankNTypes language extension. Therefore, readPrec (and its
cousin, readListPrec) are marked as GHC-only. Nevertheless, it is
recommended to use readPrec instead of readsPrec whenever possible
for the efficiency improvements it brings.
As mentioned above, derived Read instances in GHC will implement
readPrec instead of readsPrec. The default implementations of
readsPrec (and its cousin, readList) will simply use readPrec under
the hood. If you are writing a Read instance by hand, it is recommended
to write it like so:
instanceReadT wherereadPrec= ...readListPrec=readListPrecDefault
Instances
| Read Bool | Since: 2.1 |
| Read Char | Since: 2.1 |
| Read Double | Since: 2.1 |
| Read Float | Since: 2.1 |
| Read Int | Since: 2.1 |
| Read Integer | Since: 2.1 |
| Read Ordering | Since: 2.1 |
| Read Word | Since: 4.5.0.0 |
| Read () | Since: 2.1 |
| Read Lexeme | Since: 2.1 |
| Read GeneralCategory | |
| Read a => Read [a] | Since: 2.1 |
| Read a => Read (Maybe a) | Since: 2.1 |
| (Integral a, Read a) => Read (Ratio a) | Since: 2.1 |
| Read a => Read (Identity a) | This instance would be equivalent to the derived instances of the
Since: 4.8.0.0 |
| (Read b, Read a) => Read (Either a b) | |
| (Read a, Read b) => Read (a, b) | Since: 2.1 |
| (Ix a, Read a, Read b) => Read (Array a b) | Since: 2.1 |
| (Read a, Read b, Read c) => Read (a, b, c) | Since: 2.1 |
| (~) k a b => Read ((:~:) k a b) | Since: 4.7.0.0 |
| (Read e, Read1 m, Read a) => Read (ErrorT e m a) | |
| (Read a, Read b, Read c, Read d) => Read (a, b, c, d) | Since: 2.1 |
| (~~) k1 k2 a b => Read ((:~~:) k1 k2 a b) | Since: 4.10.0.0 |
| (Read a, Read b, Read c, Read d, Read e) => Read (a, b, c, d, e) | Since: 2.1 |
| (Read a, Read b, Read c, Read d, Read e, Read f) => Read (a, b, c, d, e, f) | Since: 2.1 |
| (Read a, Read b, Read c, Read d, Read e, Read f, Read g) => Read (a, b, c, d, e, f, g) | Since: 2.1 |
| (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) | Since: 2.1 |
| (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) | Since: 2.1 |
| (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) | Since: 2.1 |
| (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) | Since: 2.1 |
| (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) | Since: 2.1 |
| (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) | Since: 2.1 |
| (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) | Since: 2.1 |
| (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) | Since: 2.1 |
Conversion of values to readable Strings.
Derived instances of Show have the following properties, which
are compatible with derived instances of Read:
- The result of
showis 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
showsPrecwill produce infix applications of the constructor. - the representation will be enclosed in parentheses if the
precedence of the top-level constructor in
xis less thand(associativity is ignored). Thus, ifdis0then the result is never surrounded in parentheses; ifdis11it is always surrounded in parentheses, unless it is an atomic expression. - If the constructor is defined using record syntax, then
showwill 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 = 5Note that right-associativity of :^: is ignored. For example,
show(Leaf 1 :^: Leaf 2 :^: Leaf 3)"Leaf 1 :^: (Leaf 2 :^: Leaf 3)".
Instances
| Show Bool | |
| Show Char | Since: 2.1 |
| Show Int | Since: 2.1 |
| Show Integer | Since: 2.1 |
| Show Ordering | |
| Show Word | Since: 2.1 |
| Show CallStack | Since: 4.9.0.0 |
| Show SomeTypeRep | Since: 4.10.0.0 |
| Show () | |
| Show TyCon | Since: 2.1 |
| Show Module | Since: 4.9.0.0 |
| Show TrName | Since: 4.9.0.0 |
| Show DataType | |
| Show Constr | Since: 4.0.0.0 |
| Show DataRep | |
| Show ConstrRep | |
| Show Fixity | |
| Show SrcLoc | |
| Show CompileState | |
| Show CompileWriter | |
| Show Rational # | |
| Show Day # | |
| Show UTCTime # | |
| Show a => Show [a] | Since: 2.1 |
| Show a => Show (Maybe a) | |
| Show a => Show (Ratio a) | Since: 2.0.1 |
| Show a => Show (Identity a) | This instance would be equivalent to the derived instances of the
Since: 4.8.0.0 |
| Show l => Show (PragmasAndModuleName l) | |
| Show l => Show (PragmasAndModuleHead l) | |
| Show l => Show (ModuleHeadAndImports l) | |
| Show a => Show (NonGreedy a) | |
| Show a => Show (ListOf a) | |
| (Show b, Show a) => Show (Either a b) | |
| Show (TypeRep k a) | |
| (Show a, Show b) => Show (a, b) | Since: 2.1 |
| (Show a, Show b, Show c) => Show (a, b, c) | Since: 2.1 |
| Show ((:~:) k a b) | |
| (Show e, Show1 m, Show a) => Show (ErrorT e m a) | |
| (Show a, Show b, Show c, Show d) => Show (a, b, c, d) | Since: 2.1 |
| Show ((:~~:) k1 k2 a b) | Since: 4.10.0.0 |
| (Show a, Show b, Show c, Show d, Show e) => Show (a, b, c, d, e) | Since: 2.1 |
| (Show a, Show b, Show c, Show d, Show e, Show f) => Show (a, b, c, d, e, f) | Since: 2.1 |
| (Show a, Show b, Show c, Show d, Show e, Show f, Show g) => Show (a, b, c, d, e, f, g) | Since: 2.1 |
| (Show a, Show b, Show c, Show d, Show e, Show f, Show g, Show h) => Show (a, b, c, d, e, f, g, h) | Since: 2.1 |
| (Show a, Show b, Show c, Show d, Show e, Show f, Show g, Show h, Show i) => Show (a, b, c, d, e, f, g, h, i) | Since: 2.1 |
| (Show a, Show b, Show c, Show d, Show e, Show f, Show g, Show h, Show i, Show j) => Show (a, b, c, d, e, f, g, h, i, j) | Since: 2.1 |
| (Show a, Show b, Show c, Show d, Show e, Show f, Show g, Show h, Show i, Show j, Show k) => Show (a, b, c, d, e, f, g, h, i, j, k) | Since: 2.1 |
| (Show a, Show b, Show c, Show d, Show e, Show f, Show g, Show h, Show i, Show j, Show k, Show l) => Show (a, b, c, d, e, f, g, h, i, j, k, l) | Since: 2.1 |
| (Show a, Show b, Show c, Show d, Show e, Show f, Show g, Show h, Show i, Show j, Show k, Show l, Show m) => Show (a, b, c, d, e, f, g, h, i, j, k, l, m) | Since: 2.1 |
| (Show a, Show b, Show c, Show d, Show e, Show f, Show g, Show h, Show i, Show j, Show k, Show l, Show m, Show n) => Show (a, b, c, d, e, f, g, h, i, j, k, l, m, n) | Since: 2.1 |
| (Show a, Show b, Show c, Show d, Show e, Show f, Show g, Show h, Show i, Show j, Show k, Show l, Show m, Show n, Show o) => Show (a, b, c, d, e, f, g, h, i, j, k, l, m, n, o) | Since: 2.1 |
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.
Instances
| Eq Bool | |
| Eq Char | |
| Eq Double | |
| Eq Float | |
| Eq Int | |
| Eq Integer | |
| Eq Ordering | |
| Eq Word | |
| Eq SomeTypeRep | |
| Eq () | |
| Eq TyCon | |
| Eq Module | |
| Eq TrName | |
| Eq BigNat | |
| Eq Constr | Equality of constructors Since: 4.0.0.0 |
| Eq DataRep | |
| Eq ConstrRep | |
| Eq Fixity | |
| Eq SrcLoc | |
| Eq Text # | |
| Eq Day # | |
| Eq UTCTime # | |
| Eq a => Eq [a] | |
| Eq a => Eq (Maybe a) | |
| Eq a => Eq (Ratio a) | |
| Eq a => Eq (Identity a) | |
| Eq l => Eq (PragmasAndModuleName l) | |
| Eq l => Eq (PragmasAndModuleHead l) | |
| Eq l => Eq (ModuleHeadAndImports l) | |
| Eq a => Eq (NonGreedy a) | |
| Eq a => Eq (ListOf a) | |
| (Eq b, Eq a) => Eq (Either a b) | |
| Eq (TypeRep k a) | Since: 2.1 |
| (Eq a, Eq b) => Eq (a, b) | |
| (Eq a, Eq b, Eq c) => Eq (a, b, c) | |
| Eq ((:~:) k a b) | |
| (Eq e, Eq1 m, Eq a) => Eq (ErrorT e m a) | |
| (Eq a, Eq b, Eq c, Eq d) => Eq (a, b, c, d) | |
| Eq ((:~~:) k1 k2 a b) | Since: 4.10.0.0 |
| (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) | |
The Maybe type encapsulates an optional value. A value of type
Maybe aa (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.
Instances
| Monad Maybe | Since: 2.1 |
| Functor Maybe | Since: 2.1 |
| Applicative Maybe | Since: 2.1 |
| Alternative Maybe | Since: 2.1 |
| MonadPlus Maybe | Since: 2.1 |
| Eq1 Maybe | Since: 4.9.0.0 |
| Ord1 Maybe | Since: 4.9.0.0 |
| Read1 Maybe | Since: 4.9.0.0 |
| Show1 Maybe | Since: 4.9.0.0 |
| Eq a => Eq (Maybe a) | |
| Data a => Data (Maybe a) | Since: 4.0.0.0 |
| Ord a => Ord (Maybe a) | |
| Read a => Read (Maybe a) | Since: 2.1 |
| Show a => Show (Maybe a) | |
| Monoid a => Monoid (Maybe a) | Lift a semigroup into Since: 2.1 |
| type (==) (Maybe k) a b | |
sequence :: [Fay a] -> Fay [a] Source #
Evaluate each action in the sequence from left to right, and collect the results.
enumFromTo :: (Ord t, Num t) => t -> t -> [t] Source #
enumFromBy :: Num t => t -> t -> [t] Source #
enumFromThen :: Num t => t -> t -> [t] Source #
enumFromByTo :: (Ord t, Num t) => t -> t -> t -> [t] Source #
enumFromThenTo :: (Ord t, Num t) => t -> t -> t -> [t] Source #
fromIntegral :: (Num a, Num b) => Ptr a -> Ptr b Source #
fromInteger :: Num a => Ptr Integer -> Ptr a Source #
data Either a b :: * -> * -> * #
The Either type represents values with two possibilities: a value of
type Either a bLeft aRight 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").
Examples
The type Either String IntString or an Int. The Left constructor can be used only on
Strings, and the Right constructor can be used only on Ints:
>>>let s = Left "foo" :: Either String Int>>>sLeft "foo">>>let n = Right 3 :: Either String Int>>>nRight 3>>>:type ss :: Either String Int>>>:type nn :: Either String Int
The fmap from our Functor instance will ignore Left values, but
will apply the supplied function to values contained in a Right:
>>>let s = Left "foo" :: Either String Int>>>let n = Right 3 :: Either String Int>>>fmap (*2) sLeft "foo">>>fmap (*2) nRight 6
The Monad instance for Either allows us to chain together multiple
actions which may fail, and fail overall if any of the individual
steps failed. First we'll write a function that can either parse an
Int from a Char, or fail.
>>>import Data.Char ( digitToInt, isDigit )>>>:{let parseEither :: Char -> Either String Int parseEither c | isDigit c = Right (digitToInt c) | otherwise = Left "parse error">>>:}
The following should work, since both '1' and '2' can be
parsed as Ints.
>>>:{let parseMultiple :: Either String Int parseMultiple = do x <- parseEither '1' y <- parseEither '2' return (x + y)>>>:}
>>>parseMultipleRight 3
But the following should fail overall, since the first operation where
we attempt to parse 'm' as an Int will fail:
>>>:{let parseMultiple :: Either String Int parseMultiple = do x <- parseEither 'm' y <- parseEither '2' return (x + y)>>>:}
>>>parseMultipleLeft "parse error"
Instances
| Eq2 Either | Since: 4.9.0.0 |
| Ord2 Either | Since: 4.9.0.0 |
| Read2 Either | Since: 4.9.0.0 |
| Show2 Either | Since: 4.9.0.0 |
| Monad (Either e) | Since: 4.4.0.0 |
| Functor (Either a) | Since: 3.0 |
| Applicative (Either e) | Since: 3.0 |
| Eq a => Eq1 (Either a) | Since: 4.9.0.0 |
| Ord a => Ord1 (Either a) | Since: 4.9.0.0 |
| Read a => Read1 (Either a) | Since: 4.9.0.0 |
| Show a => Show1 (Either a) | Since: 4.9.0.0 |
| (Eq b, Eq a) => Eq (Either a b) | |
| (Data a, Data b) => Data (Either a b) | Since: 4.0.0.0 |
| (Ord b, Ord a) => Ord (Either a b) | |
| (Read b, Read a) => Read (Either a b) | |
| (Show b, Show a) => Show (Either a b) | |
| type (==) (Either k1 k2) a b | |
The value of seq a b is bottom if a is bottom, and
otherwise equal to b. seq is usually introduced to
improve performance by avoiding unneeded laziness.
A note on evaluation order: the expression seq a b does
not guarantee that a will be evaluated before b.
The only guarantee given by seq is that the both a
and b will be evaluated before seq returns a value.
In particular, this means that b may be evaluated before
a. If you need to guarantee a specific order of evaluation,
you must use the function pseq from the "parallel" package.
properFraction :: Double -> (Int, Double) Source #
Implemented in Fay, not fast.
intersperse :: a -> [a] -> [a] Source #
prependToAll :: a -> [a] -> [a] Source #
intercalate :: [a] -> [[a]] -> [a] Source #
ifThenElse :: Bool -> t -> t -> t Source #
Default definition for using RebindableSyntax.