\documentclass[11pt]{article} %include lhs2TeX.fmt \usepackage[T1]{fontenc} \usepackage[sc]{mathpazo} \linespread{1.05} \usepackage{helvet} \usepackage{multicol} \usepackage[landscape, top=0.2in, bottom=1in, left=0.2in, right=0.2in, dvips]{geometry} \usepackage{verbatim} \usepackage{url} \usepackage{fancyhdr} \pagestyle{fancy} \fancyhf{} \lfoot{\copyright\ 2008 Justin Bailey.} \cfoot{\thepage} \rfoot{\url{jgbailey@@codeslower.com}} \renewcommand\footrulewidth{0.4pt} \newcommand{\hd}[1]{\section*{\textsf #1}} \newcommand{\shd}[1]{\subsection*{\textsf{#1}}} \newcommand{\sshd}[1]{\medskip\noindent{\bfseries\textsf #1}\hspace{\parindent}} \begin{document} \begin{multicols}{3} \section*{\textsf{\LARGE Haskell Cheat Sheet\normalsize}} This cheat sheet attempts to lay out the fundamental elements of the Haskell language and libraries. It should serve as a reference to both those learning Haskell and those who are familiar with it, but maybe can't remember all the varieties of syntax and functionality. It is presented as both an executable Haskell file and a printable document. Load the source into your favorite interpreter to play with code samples shown. \begin{comment}
> {-# LANGUAGE MultiParamTypeClasses #-}
> module CheatSheet where
> import Data.Char (isUpper, isLower, toUpper, toLower, isSpace)
> import System.IO (readFile)
> import System.Directory (doesFileExist)
> import qualified Data.Set as Set
> import qualified Data.Char as Char
\end{comment} \hd{Syntax} Below the most basic syntax for Haskell is given. \shd{Comments} A single line comment starts with `@--@' and extends to the end of the line. Multi-line comments start with '@{-@' and extend to '@-}@'. Comments can be nested. Comments above function definitions should start with `@{- |@' and those next to parameter types with `@-- ^@' for compatibility with Haddock, a system for documenting Haskell code. \shd{Reserved Words} The following lists the reserved words defined by Haskell. It is a syntax error to give a variable or function one of these names. < case, class, data, deriving, do, < else, if, import, in, infix, infixl, < infixr, instance, let, of, module, < newtype, return, then, type, where \shd{Strings} @"abc"@ -- Unicode string.\\ @'a'@ -- Single character. \sshd{Multi-line Strings} Normally, it is syntax error if a string has any actual new line characters. That is, this is a syntax error: < string1 = "My long < string." However, backslashes (`@\@') can be used to ``escape'' around the new line:
> string1 = "My long \
> \string."
The area between the backslashes is ignored. An important note is that new lines \emph{in} the string must still be represented explicitly:
> string2 = "My long \n\
> \string."
That is, @string1@ evaluates to: < My long string. While @string2@ evaluates to: < My long < string. \shd{Numbers} @1@ - Integer\\ @1.0, 1e10@ - Floating point\\ @[1..10]@ -- List of numbers -- $1, 2, ... 10$\\ @[100..]@ -- Infinite list of numbers -- $100, 101, 102, ... $\\ @[110..100]@ -- Empty list; ranges only go forwards.\\ @[0, -1 ..]@ -- Negative integers.\\ @[-100..-110]@ -- Syntax error; need [-100.. -110] for negatives.\\ @[1,3..100], [-1,3..100]@ -- List from 1 to 100 by 2, -1 to 100 by 4. \shd{Lists \& Tuples} @[]@ -- Empty list.\\ @[1,2,3]@ -- List of three numbers.\\ @1 : 2 : 3 : []@ -- Alternate way to write lists using ``cons'' (@:@) and ``nil'' (@[]@).\\ @"abc"@ -- List of three characters (strings are lists).\\ @'a' : 'b' : 'c' : []@ -- List of characters (same as @"abc"@).\\ @(1,"a")@ -- 2-element tuple of a number and a string.\\ @(head, tail, 3, 'a')@ -- 4-element tuple of two functions, a number and a character. \shd{``Layout'' rule, braces and semi-colons.} Haskell can be written using braces and semi-colons, just like C. However, no one does. Instead, the ``layout'' rule is used, where spaces represent scope. The general rule is -- always indent. When the compiler complains, indent more. \sshd{Braces and semi-colons} Semi-colons terminate an expression, and braces represent scope: < < square x = { x * x; } < \sshd{Function Definition} Indent the body at least one space from the function name: < square x = < x * x Unless a @where@ clause is present. In that case, indent the where clause at least one space from the function name and any function bodies at least one space from the @where@ keyword: < square x = < x2 < where x2 = < x * x \sshd{Let} Indent the body of the let at least one space from the first definition in the @let@. If @let@ appears on its own line, the body of any defintion must appear in the column after the let: < square x = < let x2 = < x * x < in x2 As can be seen above, the @in@ keyword must also be in the same column as @let@. Finally, when multiple defintions are given, all identifiers must appear in the same column. \hd{Keywords} Haskell keywords are listed below, in alphabetical order. \shd{Case} @case@ is similar to a @switch@ statement in C\# or Java, but can take action based on any possible value for the type of the value being inspected. Consider a simple data type such as the following:
> data Choices = First String | Second |
>   Third | Fourth
>   deriving (Show, Eq)
\end{comment} \noindent @case@ can be used to determine which choice was given:
> whichChoice ch =
>   case ch of
>     First _ -> "1st!"
>     Second -> "2nd!"
>     _ -> "Something else."
As with pattern-matching in function definitions, the `@_@' character is a ``wildcard'' and matches any value. \sshd{Nesting \& Capture} Nested matching and argument capture are also allowed. Recalling the definition of @Maybe@ above, we can determine if any choice was given using a nested match:
> anyChoice1 ch =
>   case ch of
>     Nothing -> "No choice!"
>     Just (First _) -> "First!"
>     Just Second -> "Second!"
>     _ -> "Something else."
We can use argument capture to display the value matched if we wish:%
> anyChoice2 ch =
>   case ch of
>     Nothing -> "No choice!"
>     Just score@(First "gold") ->
>       "First with gold!"
>     Just score@(First _) ->
>       "First with something else: "
>         ++ show score
>     _ -> "Not first."
\sshd{Matching Order} Matching proceeds from top to bottom. If we re-wrote @anyChoice1@ as below, we'll never know what choice was actually given because the first pattern will always succeed:
> anyChoice3 ch =
>   case ch of
>     _ -> "Something else."
>     Nothing -> "No choice!"
>     Just (First _) -> "First!"
>     Just Second -> "Second!"
\sshd{Guards} Guards, or conditional matches, can be used in cases just like function definitions. The only difference is the use of the @->@ instead of @=@. Here is a simple function which does a case-insensitive string match:
> strcmp [] [] = True
> strcmp s1 s2 = case (s1, s2) of
>   (s1:ss1, s2:ss2)
>     | toUpper s1 == toUpper s2 ->
>         strcmp ss1 ss2
>     | otherwise -> False
>   _ -> False
\shd{Class} A Haskell function is defined to work on a certain type or set of types and cannot be defined more than once. Most languages support the idea of ``overloading'', where a function can have different behavior depending on the type of its arguments. Haskell accomplishes overloading through @class@ and @instance@ declarations. A @class@ defines one or more functions that can be applied to any types which are members (i.e., instances) of that class. A class is analagous to an interface in Java or C#, and instances to a concrete implementation of the interface. A class must be declared with one or more type variables. Technically, Haskell 98 only allows one type variable, but most implementations of Haskell support so-called \emph{multi-parameter type classes}, which allow more than one type variable. We can define a class which supplies a flavor for a given type:
> class Flavor a where
>   flavor :: a -> String
Notice that the declaration only gives the type signature of the function - no implementation is given here (with some exceptions, see ``Defaults'' below). Continuing, we can define several instances:
> instance Flavor Bool where
>   flavor _ = "sweet"
> instance Flavor Char where
>   flavor _ = "sour"
Evaluating @flavor True@ gives: < > flavor True < "sweet" While @flavor 'x'@ gives: < > flavor 'x' < "sour" \sshd{Defaults} Default implementations can be given for functions in a class. These are useful when certain functions can be defined in terms of others in the class. A default is defined by giving a body to one of the member functions. The canonical example is @Eq@, which can defined @/=@ (not equal) in terms of @==@. : < class Eq a where < (==) :: a -> a -> Bool < (/=) :: a -> a -> Bool < (/=) a b = not (a == b) In fact, recursive definitions can be created, but one class member must always be implemented by any @instance@ declarations. \shd{Data} So-called \emph{algebraic data types} can be declared as follows:
> data MyType = MyValue1 | MyValue2
>   deriving (Show, Eq)
\end{comment} @MyType@ is the type's \emph{name}. @MyValue1@ and @MyValue@ are \emph{values} of the type and are called \emph{constructors}. Multiple constructors are separated with the `||' character. Note that type and constructor names \emph{must} start with a capital letter. It is a syntax error otherwise. \sshd{Constructors with Arguments} The type above is not very interesting except as an enumeration. Constructors that take arguments can be declared, allowing more information to be stored with your type:
> data Point = TwoD Int Int
>   | ThreeD Int Int Int
Notice that the arguments for each constructor are \emph{type} names, not constructors. That means this kind of declaration is illegal: < data Poly = Triangle TwoD TwoD TwoD instead, the @Point@ type must be used:
> data Poly = Triangle Point Point Point
\sshd{Type and Constructor Names} Type and constructor names can be the same, because they will never be used in a place that would cause confusion. For example:
> data User = User String | Admin String
which declares a type named @User@ with two constructors, @User@ and @Admin@. Using this type in a function makes the difference clear:
> whatUser (User _) = "normal user."
> whatUser (Admin _) = "admin user."
Some literature refers to this practice as \emph{type punning}. \sshd{Type Variables} Declaring so-called \emph{polymorphic} data types is as easy as adding type variables in the declaration:
> data Slot1 a = Slot1 a | Empty1
This declares a type @Slot1@ with two constructors, @Slot1@ and @Empty1@. The @Slot1@ constructor can take an argument of \emph{any} type, which is reprented by the type variable @a@ above. We can also mix type variables and specific types in constructors:
> data Slot2 a = Slot2 a Int | Empty2
Above, the @Slot2@ constructor can take a value of any type and an @Int@ value. \sshd{Record Syntax} Constructor arguments can be declared either positionally, as above, or using record syntax, which gives a name to each argument. For example, here we declare a @Contact@ type with names for appropriate arguments:
> data Contact = Contact { ctName :: String
>       , ctEmail :: String
>       , ctPhone :: String }
These names are referred to as \emph{selector} or \emph{accessor} functions and are just that, functions. They must start with a lowercase letter or underscore and cannot have the same name as another function in scope. Thus the ``@ct@'' prefix on each above. Multiple constructors (of the same type) can use the same accessor function for values of the same type, but that can be dangerous if the accessor is not used by all constructors. Consider this rather contrived example:
> data Con = Con { conValue :: String }
>   | Uncon { conValue :: String }
>   | Noncon
> whichCon con = "convalue is " ++
>   conValue con
If @whichCon@ is called with a @Noncon@ value, a runtime error will occur. Finally, as explained elsewhere, these names can be used for pattern matching, argument capture and ``updating.'' \sshd{Class Constraints} Data types can be declared with class constraints on the type variables, but this practice is generally discouraged. It is generally better to hide the ``raw'' data constructors using the module system and instead export ``smart'' constructors which apply appropriate constraints. In any case, the syntax used is:
> data (Num a) => SomeNumber a = Two a a
>   | Three a a a
This declares a type @SomeNumber@ which has one type variable argument. Valid types are those in the @Num@ class. \sshd{Deriving} Many types have common operations which are tediuos to define yet very necessary, such as the ability to convert to and from strings, compare for equality, or order in a sequence. These capabilities are defined as typeclasses in Haskell. Because seven of these operations are so common, Haskell provides the @deriving@ keyword which will automatically implement the typeclass on the associated type. The seven supported typeclasses are: @Eq@, @Read@, @Show@, @Ord@, @Enum@, @Ix@, and @Bounded@. Two forms of @deriving@ are possible. The first is used when a type only derives on class:
> data Priority = Low | Medium | High
>   deriving Show
The second is used when multiple classes are derived:
> data Alarm = Soft | Loud | Deafening
>   deriving (Read, Show)
It is a syntax error to specify @deriving@ for any other classes besides the six given above. \shd{Deriving} See the section on @deriving@ under the @data@ keyword above. \shd{Do} The @do@ keyword indicates that the code to follow will be in a \emph{monadic} context. Statements are separated by newlines, assignment is indicated by @<-@, and a @let@ form is introduce which does not require the @in@ keyword. \sshd{If and IO} @if@ is tricky when used with IO. Conceptually it is are no different, but intuitively it is hard to deal with. Consider the function @doesFileExists@ from @System.Directory@: < doesFileExist :: FilePath -> IO Bool The @if@ statement has this ``signature'': < if-then-else :: Bool -> a -> a -> a That is, it takes a @Bool@ value and evaluates to some other value based on the condition. From the type signatures it is clear that @doesFileExist@ cannot be used directly by @if@: < wrong fileName = < if doesFileExist fileName < then ... < else ... That is, @doesFileExist@ results in an @IO Bool@ value, while @if@ wants a @Bool@ value. Instead, the correct value must be ``extracted'' by running the IO action:
> right1 fileName = do
>   exists <- doesFileExist fileName
>   if exists
>     then return 1
>     else return 0
Notice the use of @return@, too. Because @do@ puts us ``inside'' the @IO@ monad, we can't ``get out'' except through @return@. Note that we don't have to use @if@ inline here - we can also use @let@ to evaluate the condition and get a value first:
> right2 fileName = do
>   exists <- doesFileExist fileName
>   let result =
>         if exists
>           then 1
>           else 0
>   return result
Again, notice where @return@ is. We don't put it in the @let@ statement. Instead we use it once at the end of the function. \sshd{Multiple @do@'s} When using @do@ with @if@ or @case@, another @do@ is required if either branch has multiple statements. An example with @if@:
> countBytes1 f =
>   do
>     putStrLn "Enter a filename."
>     args <- getLine
>     if length args == 0
>       -- no 'do'.
>       then putStrLn "No filename given."
>       else
>         -- multiple statements require
>         -- a new 'do'.
>         do
>           f <- readFile args
>           putStrLn ("The file is " ++
>             show (length f)
>             ++ " bytes long.")
And one with @case@:
> countBytes2 =
>   do
>     putStrLn "Enter a filename."
>     args <- getLine
>     case args of
>       [] -> putStrLn "No args given."
>       file -> do
>        f <- readFile file
>        putStrLn ("The file is " ++
>          show (length f)
>          ++ " bytes long.")
An alternative is to provide semi-colons and braces. A @do@ is still required, but no indenting is needed. The below shows a @case@ example but it applies to @if@ as well:
> countBytes3 =
>   do
>     putStrLn "Enter a filename."
>     args <- getLine
>     case args of
>       [] -> putStrLn "No args given."
>       file -> do { f <- readFile file;
>        putStrLn ("The file is " ++
>          show (length f)
>          ++ " bytes long."); }
\shd{Export} See the section on @module@ below. \shd{If, Then, Else} Remember, @if@ always ``returns'' a value. It is an expression, not just a control flow statement. This function tests if the string given starts with a lower case letter and, if so, converts it to upper case:
> -- Use pattern-matching to
> -- get first character
> sentenceCase (s:rest) =
>  if isLower s
>    then toUpper s : rest
>    else s : rest
> -- Anything else is empty string
> sentenceCase _ = []
\shd{Import} See the section on @module@ below. \shd{In} See @let@. \shd{Infix, infixl and infixr} See the section on operators below. \shd{Instance} See the section on @class@ above. \shd{Let} Local functions can be defined within a function using @let@. @let@ is always followed by @in@. @in@ must appear in the same column as the @let@ keyword. Functions defined have access to all other functions and variables within the same scope (including those defined by @let@). In this example, @mult@ multiplies its argument @n@ by @x@, which was passed to the original @multiples@. @mult@ is used by map to give the multiples of x up to 10:
> multiples x =
>   let mult n = n * x
>   in map mult [1..10]
@let@ ``functions'' with no arguments are actually constants and, once evaluated, will not evaluate again. This is useful for capturing common portions of your function and re-using them. Here is a silly example which gives the sum of a list of numbers, their average, and their median:
> listStats m =
>   let numbers = [1,3 .. m]
>       total = sum numbers
>       mid = head (take (m `div` 2)
>                        numbers)
>   in "total: " ++ show total ++
>      ", mid: " ++ show mid
\sshd{Deconstruction} The left-hand side of a @let@ definition can also deconstruct its argument, in case sub-components are going to be accessed. This definition would extract the first three characters from a string
> firstThree str =
>   let (a:b:c:_) = str
>   in "Initial three characters are: " ++
>       show a ++ ", " ++
>       show b ++ ", and " ++
>       show c
Note that this is different than the following, which only works if the string has three characters:
> onlyThree str =
>   let (a:b:c) = str
>   in "The characters given are: " ++
>       show a ++ ", " ++ show b ++
>       ", and " ++ show c
\shd{Of} See the section on @case@ above. \shd{Module} A module is a compilation unit which exports functions, types, classes, instances, and other modules. A module can only be defined in one file, though its exports may come from multiple sources. To make a Haskell file a module, just add a module declaration at the top: < module MyModule where Module names must start with a capital letter but otherwise can include periods, numbers and underscores. Periods are used to give sense of structure, and Haskell compilers will use them as indications of the directory a particular source file is, but otherwise they have no meaning. The Haskell community has standardized a set of top-level module names such as @Data@, @System@, @Network@, etc. Be sure to consult them for an appropriate place for your own module if you plan on releasing it to the public. \sshd{Imports} The Haskell standard libraries are divided into a number of modules. The functionality provided by those libraries is accessed by importing into your source file. To import all everything exported by a library, just use the module name: < import Text.Read Everything means \emph{everything}: functions, data types and constructors, class declarations, and even other modules imported and then exported by the that module. Importing selectively is accomplished by giving a list of names to import. For example, here we import some functions from @Text.Read@: < import Text.Read (readParen, lex) Data types can imported in a number of ways. We can just import the type and no constructors: < import Text.Read (Lexeme) Of course, this prevents our module from pattern-matching on the values of type @Lexeme@. We can import one or more constructors explicitly: < import Text.Read (Lexeme(Ident, Symbol)) All constructors for a given type can also be imported: < import Text.Read (Lexeme(..)) We can also import types and classes defined in the module: < import Text.Read (Read, ReadS) In the case of classes, we can import the functions defined for the using syntax similar to importing constructors for data types: < import Text.Read (Read(readsPrec < , readList)) Note that, unlike data types, all class functions are imported unless explicitly excluded. To \emph{only} import the class, we use this syntax: < import Text.Read (Read()) \sshd{Exclusions} If most, but not all, names are going to imported from a module, it would be tedious to specify all those names except a few. For that reason, imports can also be specified via the @hiding@ keyword: < import Data.Char hiding (isControl < , isMark) Except for instance declarations, any type, function, constructor or class can be hidden. \sshd{Instance Declarations} It must be noted that @instance@ declarations \emph{cannot} be excluded from import. \emph{Any} @instance@ declarations in a module will be imported when the module is imported. \sshd{Qualified Imports} The names exported by a module (i.e., functions, types, operators, etc.) can have a prefix attached through qualified imports. This is particularly useful for modules which have a large number of functions having the same name as @Prelude@ functions. @Data.Set@ is a good example: < import qualified Data.Set as Set This form requires any function, type, constructor or other name exported by @Data.Set@ to now be prefixed with the \emph{alias} (i.e., @Set@) given. Here is one way to remove all duplicates from a list:
> removeDups a =
>   Set.toList (Set.fromList a)
A second form does not create an alias. Instead, the prefix becomes the module name. We can write a simple function to check if a string is all upper case: < import qualified Char
> allUpper str =
>   all Char.isUpper str
Except for the prefix specified, qualified imports support the same syntax as normal imports. The name imported can be limited in the same ways as described above. \sshd{Exports} If an export list is not provided, then all functions, types, constructors, etc. will be available to anyone importing the module. Note that any imported modules are \emph{not} exported in this case. Limiting the names exported is accomplished by adding a parenthesized list of names before the @where@ keyword: < module MyModule (MyType < , MyClass < , myFunc1 < ...) < where The same syntax as used for importing can be used here to specify which functions, types, constructors, and classes are exported, with a few differences. If a module imports another module, it can also export that module: < module MyBigModule (module Data.Set < , module Data.Char) < where < < import Data.Set < import Data.Char A module can even re-export itself, which can be useful when all local definitions and a given imported module are to be exported. Below we export ourselves and @Data.Set@, but not @Data.Char@: < module AnotherBigModule (module Data.Set < , module AnotherBigModule) < where < < import Data.Set < import Data.Char \shd{Newtype} While @data@ introduces new values and @type@ just creates synonyms, @newtype@ falls somewhere between. The syntax for @newtype@ is quite restricted -- only one constructor can be defined, and that constructor can only take one argument. Continuing the example above, we can define a @Phone@ type like the following:
> newtype Home = H String
> newtype Work = W String
> data Phone = Phone Home Work
As opposed to @type@, the @H@ and @W@ ``values'' on @Phone@ are \emph{not} just @String@ values. The typechecker treats them as entirely new types. That means our @lowerName@ function from above would not compile. The following produces a type error: < lPhone (Phone hm wk) = < Phone (lower hm) (lower wk) Instead, we must use pattern-matching to get to the ``values'' to which we apply @lower@:
> lPhone (Phone (H hm) (W wk)) =
>   Phone (H (lower hm)) (W (lower wk))
The key observation is that this keyword does not introduce a new value; instead it introduces a new type. This gives us two very useful properties: \begin{itemize} \item No runtime cost is associated with the new type, since it does not actually produce new values. In other words, newtypes are absolutely free! \item The type-checker is able to enforce that common types such as @Int@ or @String@ are used in restricted ways, specified by the programmer. \end{itemize} Finally, it should be noted that any @deriving@ clause which can be attached to a @data@ declaration can also be used when declaring a @newtype@. \shd{Return} See @do@ above. \shd{Type} This keyword defines a \emph{type synonym} (i.e., alias). This keyword does not define a new type, like @data@ or @newtype@. It is useful for documenting code but otherwise has no effect on the actual type of a given function or value. For example, a @Person@ data type could be defined as: < data Person = Person String String where the first constructor argument represents their first name and the second their last. However, the order and meaning of the two arguments is not very clear. A @type@ declaration can help:
> type FirstName = String
> type LastName = String
> data Person = Person FirstName LastName
Because @type@ introduces a synonym, type checking is not affected in any way. The function @lower@, defined as:
> lower s = map toLower s
which has the type < lower :: String -> String can be used on values with the type @FirstName@ or @LastName@ just as easily:
> lName (Person f l ) =
>   Person (lower f) (lower l)
Because @type@ is just a synonym, it can't declare multiple constructors like @data@ can. Type variables can be used, but there cannot be more than the type variables declared with the original type. That means a synonmym like the following is possible: < type NotSure a = Maybe a but this not: < type NotSure a b = Maybe a Note that \emph{fewer} type variables can be used, which useful in certain instances. \shd{Where} Similar to @let@, @where@ defines local functions and constants. The scope of a @where@ definition is the current function. If a function is broken into multiple definitions through pattern-matching, then the scope of a particular @where@ clause only applies to that definition. For example, the function @result@ below has a different meaning depending on the arguments given to the function @strlen@:
> strlen [] = result
>   where result = "No string given!"
> strlen f = result ++ " characters long!"
>   where result = show (length f)
\sshd{Where vs. Let} A @where@ clause can only be defined at the level of a function definition. Usually, that is identical to the scope of @let@ definition. The only difference is when guards are being used. The scope of the @where@ clause extends over all guards. In contrast, the scope of a @let@ expression is only the current function clause \emph{and} guard, if any. \hd{Declarations, Etc.} The following section details rules on function declarations, list comprehensions, and other areas of the language. \shd{Function Definition} Functions are defined by declaring their name, any arguments, and an equals sign:
> square x = x * x
\emph{All} functions names must start with a lowercase letter or ``@_@''. It is a syntax error otherwise. \sshd{Pattern Matching} Multiple ``clauses'' of a function can be defined by ``pattern-matching'' on the values of arguments. Here, the the @agree@ function has four separate cases:
> -- Matches when the string "y" is given.
> agree1 "y" = "Great!"
> -- Matches when the string "n" is given.
> agree1 "n" = "Too bad."
> -- Matches when string beginning
> -- with 'y' given.
> agree1 ('y':_) = "YAHOO!"
> -- Matches for any other value given.
> agree1 _ = "SO SAD."
Note that the `@_@' character is a wildcard and matches any value. Pattern matching can extend to nested values. Assuming this data declaration: < data Bar = Bil (Maybe Int) | Baz and recalling @Maybe@ is defined as: < data Maybe a = Just a | Nothing we can match on nested @Maybe@ values when @Bil@ is present: < f (Bil (Just _)) = ... < f (Bil Nothing) = ... < f Baz = ... Pattern-matching also allows values to be assigned to variables. For example, this function determines if the string given is empty or not. If not, the value captures in @str@ is converted to to lower case:
> toLowerStr [] = []
> toLowerStr str = map toLower str
In reality, @str@ is the same as @_@ in that it will match anything, except the value matched is also given a name. \sshd{{\ensuremath $n + k$} Patterns} This sometimes controversial pattern-matching facility makes it easy to match certain kinds of numeric expressions. The idea is to define a base case (the ``$n$'' portion) with a constant number for matching, and then to define other matches (the ``$k$'' portion) as additives to the base case. Here is a rather inefficient way of testing if a number is even or not:
> isEven 0 = True
> isEven 1 = False
> isEven (n + 2) = isEven n
\sshd{Argument Capture} Argument capture is useful for pattern-matching a value AND using it, without declaring an extra variable. Use an |@| symbol in between the pattern to match and the variable to assign the value to. This facility is used below to capture the head of the list in @l@ for display, while also capturing the entire list in @ls@ in order to compute its length:
> len ls@(l:_) = "List starts with " ++
>   show l ++ " and is " ++
>   show (length ls) ++ " items long."
> len [] = "List is empty!"
\sshd{Guards} Boolean functions can be used as ``guards'' in function definitions along with pattern matching. An example without pattern matching:
> which n
>   | n == 0 = "zero!"
>   | even n = "even!"
>   | otherwise = "odd!"
Notice @otherwise@ -- it always evaulates to true and can be used to specify a ``default'' branch. Guards can be used with patterns. Here is a function that determines if the first character in a string is upper or lower case:
> what [] = "empty string!"
> what (c:_)
>   | isUpper c = "upper case!"
>   | isLower c = "lower case"
>   | otherwise = "not a letter!"
\sshd{Matching \& Guard Order} Pattern-matching proceeds in top to bottom order. Similary, guard expressions are tested from top to bottom. For example, neither of these functions would be very interesting:
> allEmpty _ = False
> allEmpty [] = True
> alwaysEven n
>   | otherwise = False
>   | n `div` 2 == 0 = True
\sshd{Record Syntax} Normally pattern matching occurs based on the position of arguments in the value being matched. Types declared with record syntax, however, can match based on those record names. Given this data type:
> data Color = C { red
>   , green
>   , blue :: Int }
>   deriving (Show, Eq)
\end{comment} \noindent we can match on @green@ only:
> isGreenZero (C { green = 0 }) = True
> isGreenZero _ = False
Argument capture is possible with this syntax, though it gets clunky. Continuing the above, now define a @Pixel@ type and a function to replace values with non-zero @green@ components with all black:
> data Pixel = P Color
>   deriving (Show, Eq)
> -- Color value untouched if green is 0
> setGreen (P col@(C { green = 0 })) = P col
> setGreen _ = P (C 0 0 0)
\sshd{Lazy Patterns} This syntax, also known as \emph{irrefutable} patterns, allows pattern matches which always succeed. That means any clause using the pattern will succeed, but if it tries to actually use the matched value an error may occur. This is generally useful when an action should be taken on the \emph{type} of a particular value, even if the value isn't present. For example, define a class for default values:
> class Def a where
>   defValue :: a -> a
The idea is you give @defValue@ a value of the right type and it gives you back a default value for that type. Defining instances for basic types is easy:
> instance Def Bool where
>   defValue _ = False
> instance Def Char where
>   defValue _ = ' '
@Maybe@ is a littler trickier, because we want to get a default value for the type, but the constructor might be @Nothing@. The following definition would work, but it's not optimal since we get @Nothing@ when @Nothing@ is passed in. < instance Def a => Def (Maybe a) where < defValue (Just x) = Just (defValue x) < defValue Nothing = Nothing We'd rather get a {\tt Just (\rm\emph{default value}\tt)\rm} back instead. Here is where a lazy pattern saves us -- we can pretend that we've matched @Just x@ and use that to get a default value, even if @Nothing@ is given:
> instance Def a => Def (Maybe a) where
>   defValue ~(Just x) = Just (defValue x)
As long as the value @x@ is not actually evaluated, we're safe. None of the base types need to look at @x@ (see the ``@_@'' matches they use), so things will work just fine. One wrinkle with the above is that we must provide type annotations in the interpreter or the code when using a @Nothing@ constructor. @Nothing@ has type @Maybe a@ but, if not enough other information is available, Haskell must be told what @a@ is. Some example default values:
> -- Return "Just False"
> defMB = defValue (Nothing :: Maybe Bool)
> -- Return "Just ' '"
> defMC = defValue (Nothing :: Maybe Char)
\shd{List Comprehensions} A list comprehension consists of three types of elements - \emph{generators}, \emph{guards}, and \emph{targets}. A list comprehension creates a list of target values based on the generators and guards given. This comprehension generates all squares:
> squares = [x * x | x <- [1..]]
@x <- [1..]@ generates a list of all @Integer@ values and puts them in @x@, one by one. @x * x@ creates each element of the list by multiplying @x@ by itself. Guards allow certain elements to be excluded. The following shows how divisors for a given number (excluding itself) can be calculated. Notice how @d@ is used in both the guard and target expression.
> divisors n =
>   [d | d <- [1..(n `div` 2)]
>      , n `mod` d == 0]
Comprehensions are not limited to numbers. Any list will do. All upper case letters can be generated:
> ups =
>   [c | c <- [minBound .. maxBound]
>      , isUpper c]
Or to find all occurrences of a particular break value @br@ in a list @word@ (indexing from 0):
> idxs word br =
>   [i | (i, c) <- zip [0..] word
>       , c == br]
A unique feature of list comprehensions is that pattern matching failures do not cause an error - they are just excluded from the resulting list. \shd{Operators} There are very few predefined ``operators'' in Haskell - most that do look predefined are actually syntax (e.g., ``@=@''). Instead, operators are simply functions that take two arguments and have special syntax support. Any so-called operator can be applied as a normal function using parentheses: < 3 + 4 == (+) 3 4 To define a new operator, simply define it as a normal function, except the operator appears between the two arguments. Here's one which takes inserts a comma between two strings and ensures no extra spaces appear:
> first ## last =
>   let trim s = dropWhile isSpace
>         (reverse (dropWhile isSpace
>           (reverse s)))
>   in trim last ++ ", " ++ trim first
< > " Haskell " ## " Curry " < Curry, Haskell Of course, full pattern matching, guards, etc. are available in this form. Type signatures are a bit different, though. The operator ``name'' must appear in parenetheses:
> (##) :: String -> String -> String
Allowable symbols which can be used to define operators are: < # $ % & * + . / < = > ? @ \ ^ | - ~ However, there are several ``operators'' which cannot be redefined. Those are: \textt{<- -> =} (\emph{by itself}) \sshd{Precedence \& Associativity} The precedence and associativity, collectively called \emph{fixity}, of any operator can be set through the @infix@, @infixr@ and @infixl@ keywords. These can be applied both to top-level functions and to local definitions. The syntax is: \bigskip \texttt{infix} || \texttt{infixr} || \texttt{infixl} \emph{precedence} \emph{op} \bigskip \noindent where \emph{precedence} varies from 0 to 9. \emph{Op} can actually be any function which takes two arguments (i.e., any binary operation). Whether the operator is left or right associative is specified by @infixl@ or @infixr@, respectively. @infix@ declarations have no associativity. Precedence and associativity make many of the rules of arithmetic work ``as expected.'' For example, consider these minor updates to the precedence of addition and multiplication:
> infixl 8 `plus1`
> plus1 a b = a + b
> infixl 7 `mult1`
> mult1 a b = a * b
The results are surprising: < > 2 + 3 * 5 < 17 < > 2 `plus1` 3 `mult1` 5 < 25 Reversing associativy also has interesting effects. Redefining division as right associative:
> infixr 7 `div1`
> div1 a b = a / b
We get interesting results: < > 20 / 2 / 2 < 5.0 < > 20 `div1` 2 `div1` 2 < 20.0 \shd{Currying} In Haskell, functions do not have to get all of their arguments at once. For example, consider the @convertOnly@ function, which only converts certain elements of string depending on a test:
> convertOnly test change str =
>     map (\c -> if test c
>                 then change c
>                 else c) str
Using @convertOnly@, we can write the @l33t@ function which converts certain letters to numbers:
> l33t = convertOnly isL33t toL33t
>   where
>     isL33t 'o' = True
>     isL33t 'a' = True
>     -- etc.
>     isL33t _ = False
>     toL33t 'o' = '0'
>     toL33t 'a' = '4'
>     -- etc.
>     toL33t c = c
Notice that @l33t@ has no arguments specified. Also, the final argument to @convertOnly@ is not given. However, the type signature of @l33t@ tells the whole story: < l33t :: String -> String That is, @l33t@ takes a string and produces a string. It is a ``constant'', in the sense that @l33t@ always returns a value that is a function which takes a string and produces a string. @l33t@ returns a ``curried'' form of @convertOnly@, where only two of its three arguments have been supplied. This can be taken further. Say we want to write a function which only changes upper case letters. We know the test to apply, @isUpper@, but we don't want to specify the conversion. That function can be written as:
> convertUpper = convertOnly isUpper
which has the type signature: < convertUpper :: (Char -> Char) < -> String -> String That is, @convertUpper@ can take two arguments. The first is the conversion function which converts individual characters and the second is the string to be converted. A curried form of any function which takes multiple arguments can be created. One way to think of this is that each ``arrow'' in the function's signature represents a new function which can be created by supplying one more argument. \sshd{Sections} Operators are functions, and they can be curried like any other. For example, a curried version of ``@+@'' can be written as: < add10 = (+) 10 However, this can be unwieldy and hard to read. ``Sections'' are curried operators, using parentheses. Here is @add10@ using sections:
> add10 = (10 +)
The supplied argument can be on the right or left, which indicates what position it should take. This is important for operations such as concatenation:
> onLeft str = (++ str)
> onRight str = (str ++)
Which produces quite different results: < > onLeft "foo" "bar" < "barfoo" < > onRight "foo" "bar" < "foobar" \shd{``Updating'' values and record syntax} Haskell is a pure language and, as such, has no mutable state. That is, once a value is set it never changes. ``Updating'' is really a copy operation, with new values in the fields that ``changed.'' For example, using the @Color@ type defined earlier, we can write a function that sets the @green@ field to zero easily:
> noGreen1 (C r _ b) = C r 0 b
The above is a bit verbose and we can rewrite using record syntax. This kind of ``update'' only sets values for the field(s) specified and copies the rest:
> noGreen2 c = c { green = 0 }
Above, we capture the @Color@ value in @c@ and return a new @Color@ value. That value happens to have the same value for @red@ and @blue@ as @c@ and it's @green@ component is 0. We can combine this with pattern matching to set the @green@ and @blue@ fields to equal the @red@ field:
> makeGrey c@(C { red = r }) =
>   c { green = r, blue = r }
Notice we must use argument capture (``|c@|'') to get the @Color@ value and pattern matching with record syntax (``|C { red = r}|'') to get the inner @red@ field. \shd{Anonymous Functions} An anonymous function (i.e., a \emph{lambda expression} or \emph{lambda} for short), is a function without a name. They can be defined at any time like so: < \c -> (c, c) which defines a function which takes an argument and returns a tuple containing that argument in both positions. They are useful for simple functions which don't need a name. The following determines if a string has mixed case (or is all whitespace):
> mixedCase str =
>   all (\c -> isSpace c ||
>              isLower c ||
>              isUpper c) str
Of course, lambdas can be the returned from functions too. This classic returns a function which will then multiply its argument by the one originally given:
> multBy n = \m -> n * m
For example: < > let mult10 = multBy 10 < > mult10 10 < 100 \shd{Type Signatures} Haskell supports full type-inference, meaning in most cases no types have to be written down. Type signatures are still useful for at least two reasons. \begin{description} \item{\emph{Documentation}} -- Even if the compiler can figure out the types of your functions, other programmers or even yourself might not be able to later. Writing the type signatures on all top-level functions is considered very good form. \item{\emph{Specialization}} -- Typeclasses allow functions with overloading. For example, a function to negate any list of numbers has the signature: < negateAll :: Num a => [a] -> [a] However, for efficiency or other reasons you may only want to allow @Int@ types. You would accomplish that wiht a type signature: < negateAll :: [Int] -> [Int] \end{description} Type signatures can appear on top-level functions and nested @let@ or @where@ definitions. Generally this is useful for documentation, though in some case you may use it prevent polymorphism. A type signature is first the name of the item which will be typed, followed by a @::@, followed by the types. An example of this has already been seen above. Type signatures do not need to appear directly above their implementation. They can be specified anywhere in the containing module (yes, even below!). Multiple items with the same signature can also be defined together:
> pos, neg :: Int -> Int
< ...
> pos x | x < 0 = negate x
>       | otherwise = x
> neg y | y > 0 = negate y
>       | otherwise = y
\sshd{Type Annotations} Sometimes Haskell will not be able to determine what type you meant. The classic demonstration of this is the ``@show . read@'' problem: < canParseInt x = show (read x) Haskell cannot compile that function because it does not know the type of @x@. We must limit the type through an annotation:
> canParseInt x = show ((read x) :: Int)
Annotations have a similar syntax as type signatures, except they appear in-line with functions. \shd{Unit} @()@ -- ``unit'' type and ``unit'' value. The value and type that represents no useful information. \hd{Contributors} My thanks to those who contributed patches and useful suggestions: Jeff Zaroyko, Stephen Hicks, Holger Siegel, Adrian Neumann. \hd{Version} This is version 1.1 of the CheatSheet. The latest source can always be found at \url{git://github.com/m4dc4p/cheatsheet.git}. The latest version be downloaded from HackageDB\footnote{\url{http://hackage.haskell.org/cgi-bin/hackage-scripts/package/CheatSheet}}. Visit \url{http://blog.codeslower.com} for other projects and writings. \end{multicols} \end{document}