Haskell/GHC symbol search cheatsheet
Several features of Haskell/GHC have low googleability.
Because some of them are composed of symbols :)
This page is a reference collection to support search of them.
If you want to search for function symbols like ., $, >>=, <*>, ..., you can use the following search engines:
Happy Haskelling!
! : "strictness flag"
[ Haskell 2010 Language Report ]
data Vec = Vec !Int! : "bang pattern"
[ GHC User's Guide ]
f1 !x = # : "MagicHash"
[ GHC User's Guide ]
data Int = I# Int## : "OverloadedLabels"
[ GHC User's Guide ]
example = #x (Point 1 2)# : C pre-processor's directive
[ GHC User's Guide ]
#include "MachDeps.h"# : hsc2hs command's operator
[ GHC User's Guide ]
flag = #const VER_MAJORVERSION$( ) : Template Haskell’s splice syntax
[ GHC User's Guide ]
two = $(add1 1)$$( ) : Typed Template Haskell’s splice syntax
[ GHC User's Guide ]
two = $$(add1 1)%1 -> : "Linear types"
[ GHC User's Guide ]
f :: a %1 -> a ' : an identifier consists of a letter followed by zero or more letters, digits, underscores, and single quotes
[ Haskell 2010 Language Report ]
xs' = f ys' : promoted constructors are prefixed by a tick '
[ GHC User's Guide ]
type * = TYPE 'LiftedRep' '' : Template Haskell’s quotation syntax
[ GHC User's Guide ]
makeLenses ''FooBar() : "unit type"
[ Haskell 2010 Language Report ] [ Haskell 2010 Language Report ]
main :: IO ()() : "unit expression"
[ Haskell 2010 Language Report ] [ Haskell 2010 Language Report ]
return ()( ) : "section" - a convenient syntax for partial application
[ Haskell 2010 Language Report ]
add1 = (1+)(,) : the constructor for a tuple
[ Haskell 2010 Language Report ]
f x y = liftM2 (,) x y (, xxx) : "TupleSections"
[ GHC User's Guide ]
f xs = fmap (, True) xs(# #) : "unboxed tuple"
[ GHC User's Guide ]
f x y = (# x+1, y-1 #)(# | | #) : "unboxed sum"
[ GHC User's Guide ]
f :: (# Int | Bool | Char #) -> Int
f (# x | | #) = 1
f (# | True | #) = 2
f _ = 3(..) : export all of its names
[ Haskell 2010 Language Report ]
module GHC.Arr (
Ix(..),(..) : import all of its names
[ Haskell 2010 Language Report ]
import GHC.Types (Bool(..))* : the kind of ordinary types (synonym for Type and TYPE `LiftedRep)
[ Haskell 2010 Language Report ] [ GHC User's Guide ] [ GHC User's Guide ]
ghci> :kind Int
Int :: *-> : case expression
[ Haskell 2010 Language Report ]
f x = case x of
Nothing -> False
Just _ -> True-> : "view pattern"
[ GHC User's Guide ]
size (view -> Unit) = 1
size (view -> Arrow t1 t2) = size t1 + size t2-> : "function type"
[ Haskell 2010 Language Report ]
id :: a -> a. : module names are a dot-separated sequence
[ Haskell 2010 Language Report ]
import Data.Maybe
import qualified Text.Read.Lex as L
lexP = lift L.lex. : "OverloadedRecordDot"
[ GHC User's Guide ]
getResult c = c.result
getResults = map (.result). : "OverloadedRecordUpdate" (experimental)
[ GHC User's Guide ]
setYearTaken c y = c{taken.year = y}. : universal quantification
[ GHC User's Guide ]
f :: forall a. a -> [a]: : "list constructor" (cons)
[ Haskell 2010 Language Report ] [ Haskell 2010 Language Report ] [ Haskell 2010 Language Report ]
f x xs = x:xs: : an operator symbol starting with a colon is a constructor
[ Haskell 2010 Language Report ]
data NonEmpty a = a :| [a]:: : "type signature"
[ Haskell 2010 Language Report ]
id :: a -> a
id x = x:: : "expression type-signature" (type annotation)
[ Haskell 2010 Language Report ]
x = fromIntegral (maxBound::Int); : semicolon in layout rule
[ Haskell 2010 Language Report ]
f x = let a = 1; b = 2
g y = exp2
in exp1 <- : lambda-bound in do expression
[ Haskell 2010 Language Report ]
f = do
x <- getLine
putStrLn x<- : "pattern guard"
[ Haskell 2010 Language Report ]
f x
| Just y <- g x = => : context (type class constraint)
[ Haskell 2010 Language Report ]
subtract :: (Num a) => a -> a -> a
subtract x y = y - x? : "ImplicitParams"
[ GHC User's Guide ]
sort :: (?cmp :: a -> a -> Bool) => [a] -> [a]
sort = sortBy ?cmp@ : "as pattern"
[ Haskell 2010 Language Report ]
f s@(x:xs) = @ : "type application"
[ GHC User's Guide ]
f = read @Int[] : "empty list" (nil)
[ Haskell 2010 Language Report ] [ Haskell 2010 Language Report ]
null [] = True
null _ = False[ .. ] : "arithmetic sequence"
[ Haskell 2010 Language Report ]
xs = [1..10][ | <- ] : "list comprehension"
[ Haskell 2010 Language Report ]
xs = [x^2 | x <- [1..10]] [| |], [e| |], [d| |], [t| |], [p| |] : Template Haskell’s quotation syntax (expression, declaration, type, and pattern)
[ GHC User's Guide ]
add1 x = [| x + 1 |][varid| |] : Template Haskell’s quasi-quotation syntax
[ GHC User's Guide ]
greet name = [interpolate| Hello, #name! |][|| ||] : Typed Template Haskell’s quotation syntax
[ GHC User's Guide ]
add1 x = [|| x + 1 ||]_ : "wildcard pattern"
[ Haskell 2010 Language Report ]
f Red =
f Blue =
f _ =_ : unused identifiers beginning with underscore
[ GHC User's Guide ] [ Haskell 2010 Language Report ]
_w = True -- No warning: _w starts with an underscore_ : "typed hole" (expression level)
[ GHC User's Guide ]
sum xs = foldr _ 0 xs_ : "type wildcard" (type level)
[ GHC User's Guide ]
not' :: Bool -> _
not' x = not x_ : "extra-constraints wildcard"
[ GHC User's Guide ]
arbitCs :: _ => a -> String_ : "NumericUnderscores"
[ GHC User's Guide ]
million = 1_000_000\ -> : "lambda abstraction"
[ Haskell 2010 Language Report ]
add1 = \x -> x + 1\case -> : "LambdaCase"
[ GHC User's Guide ]
f = \case
Red -> 2
Blue -> 1
_ -> 0` ` : "infix notation" - an identifier enclosed in grave accents
[ Haskell 2010 Language Report ]
div10 x = x `div` 10{ } : brace in layout rule
[ Haskell 2010 Language Report ]
f x = case x of {Nothing -> False; Just _ -> True}{ } : "record syntax" (datatypes with field labels)
[ Haskell 2010 Language Report ]
data MyPoint = Point { x :: Int, y :: Int }{..} : "record wildcard"
[ GHC User's Guide ]
f Vec{..} = {-# #-} : "compiler pragma"
[ Haskell 2010 Language Report ] [ GHC User's Guide ] [ GHC User's Guide ]
{-# LANGUAGE TypeApplications #-}
{-# LANGUAGE GADTs #-}
{-# INLINE fmap #-}| : "boolean guard" (guard)
[ Haskell 2010 Language Report ]
clip255 x
| x > 255 = 255
| otherwise = x | : "MultiWayIf"
[ GHC User's Guide ]
if | x == ":q" -> quit
| isError x -> errorExit x
| otherwise -> execCommand x| : algebraic datatype declaration
[ Haskell 2010 Language Report ]
data Maybe a = Nothing | Just a| : "functional dependency"
[ GHC User's Guide ]
class Foo a b c | a b -> c where ~ : "irrefutable pattern"
[ Haskell 2010 Language Report ]
f1 ~(as,bs) =~ : lazy pattern when Strict is enabled
[ GHC User's Guide ]
{-# LANGUAGE Strict #-}
f ~x =~ : laziness flag
[ GHC User's Guide ]
data T = C ~a~ : "equality constraint"
[ GHC User's Guide ]
class (F a ~ b) => C a b where