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https://github.com/takenobu-hs/haskell-symbol-search-cheatsheet

Haskell/GHC symbol search cheatsheet
https://github.com/takenobu-hs/haskell-symbol-search-cheatsheet

ghc haskell search syntax

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Haskell/GHC symbol search cheatsheet

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README 

        



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:

  * [Hoogle search on Stackage](https://www.stackage.org)

  * [Hoogle on Haskell.org](https://hoogle.haskell.org/)



Happy Haskelling!



---

## `!` : "strictness flag"

\[ [Haskell 2010 Language Report](https://www.haskell.org/onlinereport/haskell2010/haskellch4.html#x10-680004.2) \]



```Haskell

data Vec = Vec !Int

```



---

## `!` : "bang pattern"

\[ [GHC User's Guide](https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/exts/strict.html#bang-patterns-informal) \]



```Haskell

f1 !x = 

```



---

## `#` : "MagicHash"

\[ [GHC User's Guide](https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/exts/magic_hash.html#the-magic-hash) \]



```Haskell

data Int = I# Int#

```



---

## `#` : "ExtendedLiterals"

\[ [GHC User's Guide](https://ghc.gitlab.haskell.org/ghc/doc/users_guide/exts/extended_literals.html) \]



```Haskell

f x = case x of

        123#Int8 -> False

        _        -> True

```



---

## `#` : "OverloadedLabels"

\[ [GHC User's Guide](https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/exts/overloaded_labels.html#overloaded-labels) \]



```Haskell

example = #x (Point 1 2)

```



---

## `#` : C pre-processor's directive

\[ [GHC User's Guide](https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/phases.html#options-affecting-the-c-pre-processor) \]



```Haskell

#include "MachDeps.h"

```



---

## `#` : hsc2hs command's operator

\[ [GHC User's Guide](https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/utils.html#input-syntax) \]



```Haskell

flag = #const VER_MAJORVERSION

```



---

## `$( )` : Template Haskell’s splice syntax

\[ [GHC User's Guide](https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/exts/template_haskell.html#template-haskell) \]



```Haskell

two = $(add1 1)

```



---

## `$$( )` : Typed Template Haskell’s splice syntax

\[ [GHC User's Guide](https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/exts/template_haskell.html#template-haskell) \]



```Haskell

two = $$(add1 1)

```



---

## `%1 ->` : "Linear types"

\[ [GHC User's Guide](https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/exts/linear_types.html#linear-types) \]



```Haskell

f :: a %1 -> a 

```



---

## `'` : an identifier consists of a letter followed by zero or more letters, digits, underscores, and single quotes

\[ [Haskell 2010 Language Report](https://www.haskell.org/onlinereport/haskell2010/haskellch2.html#x7-180002.4) \]



```Haskell

xs' = f ys

```



---

## `'` : promoted constructors are prefixed by a tick '

\[ [GHC User's Guide](https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/exts/data_kinds.html#datatype-promotion) \]



```Haskell

type * = TYPE 'LiftedRep

```



---

## `'` `''` : Template Haskell’s quotation syntax

\[ [GHC User's Guide](https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/exts/template_haskell.html#syntax) \]



```Haskell

makeLenses ''FooBar

```



---

## `()` : "unit type"

\[ [Haskell 2010 Language Report](https://www.haskell.org/onlinereport/haskell2010/haskellch4.html#x10-650004.1.2) \]

\[ [Haskell 2010 Language Report](https://www.haskell.org/onlinereport/haskell2010/haskellch6.html#x13-1220006.1.5) \]



```Haskell

main :: IO ()

```



---

## `()` : "unit expression"

\[ [Haskell 2010 Language Report](https://www.haskell.org/onlinereport/haskell2010/haskellch3.html#x8-380003.9) \]

\[ [Haskell 2010 Language Report](https://www.haskell.org/onlinereport/haskell2010/haskellch6.html#x13-1220006.1.5) \]



```Haskell

return ()

```



---

## `( )` : "section" - a convenient syntax for partial application

\[ [Haskell 2010 Language Report](https://www.haskell.org/onlinereport/haskell2010/haskellch3.html#x8-300003.5) \]



```Haskell

add1 = (1+)

```



---

## `(,)` : the constructor for a tuple

\[ [Haskell 2010 Language Report](https://www.haskell.org/onlinereport/haskell2010/haskellch3.html#x8-360003.8) \]



```Haskell

f x y = liftM2 (,) x y 

```



---

## `(, xxx)` : "TupleSections"

\[ [GHC User's Guide](https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/exts/tuple_sections.html#tuple-sections) \]



```Haskell

f xs = fmap (, True) xs

```



---

## `(#  #)` : "unboxed tuple"

\[ [GHC User's Guide](https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/exts/primitives.html#unboxed-tuples) \]



```Haskell

f x y = (# x+1, y-1 #)

```



---

## `(# | | #)` : "unboxed sum"

\[ [GHC User's Guide](https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/exts/primitives.html#unboxed-sums) \]



```Haskell

f :: (# Int | Bool | Char #) -> Int

f (# x | | #)    = 1

f (# | True | #) = 2

f _              = 3

```



---

## `(..)` : export all of its names

\[ [Haskell 2010 Language Report](https://www.haskell.org/onlinereport/haskell2010/haskellch5.html#x11-1000005.2) \]



```Haskell

module GHC.Arr (

        Ix(..),

```



---

## `(..)` : import all of its names 

\[ [Haskell 2010 Language Report](https://www.haskell.org/onlinereport/haskell2010/haskellch5.html#x11-1010005.3) \]



```Haskell

import GHC.Types (Bool(..))

```



---

## `*` : the kind of ordinary types (synonym for `Type` and ``TYPE `LiftedRep``)

\[ [Haskell 2010 Language Report](https://www.haskell.org/onlinereport/haskell2010/haskellch4.html#x10-640004.1.1) \]

\[ [GHC User's Guide](https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/exts/poly_kinds.html#the-kind-type) \]



```Haskell

ghci> :kind Int

Int :: *

```



---

## `->` : case expression

\[ [Haskell 2010 Language Report](https://www.haskell.org/onlinereport/haskell2010/haskellch3.html#x8-460003.13) \]



```Haskell

f x = case x of

        Nothing -> False

        Just _  -> True

```



---

## `->` : "view pattern"

\[ [GHC User's Guide](https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/exts/view_patterns.html#view-patterns) \]



```Haskell

size (view -> Unit)        = 1

size (view -> Arrow t1 t2) = size t1 + size t2

```



---

## `->` : "function type"

\[ [Haskell 2010 Language Report](https://www.haskell.org/onlinereport/haskell2010/haskellch4.html#x10-650004.1.2) \]



```Haskell

id :: a -> a

```



---

## `.` : module names are a dot-separated sequence

\[ [Haskell 2010 Language Report](https://www.haskell.org/onlinereport/haskell2010/haskellch2.html#x7-180002.4) \]



```Haskell

import Data.Maybe

import qualified Text.Read.Lex as L



lexP = lift L.lex

```



---

## `.` : "OverloadedRecordDot"

\[ [GHC User's Guide](https://downloads.haskell.org/ghc/9.2.1/docs/html/users_guide/exts/overloaded_record_dot.html#overloaded-record-dot) \]



```Haskell

getResult c = c.result



getResults = map (.result)

```



---

## `.` : "OverloadedRecordUpdate" (experimental)

\[ [GHC User's Guide](https://downloads.haskell.org/ghc/9.2.1/docs/html/users_guide/exts/overloaded_record_update.html#overloaded-record-update) \]



```Haskell

setYearTaken c y = c{taken.year = y}

```



---

## `.` : universal quantification

\[ [GHC User's Guide](https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/exts/explicit_forall.html#explicit-universal-quantification-forall) \]



```Haskell

f :: forall a. a -> [a]

```



---

## `:` : "list constructor" (cons)

\[ [Haskell 2010 Language Report](https://www.haskell.org/onlinereport/haskell2010/haskellch3.html#x8-340003.7) \]

\[ [Haskell 2010 Language Report](https://www.haskell.org/onlinereport/haskell2010/haskellch6.html#x13-1200006.1.3) \]

\[ [Haskell 2010 Language Report](https://www.haskell.org/onlinereport/haskell2010/haskellch2.html#x7-180002.4) \]



```Haskell

f x xs = x:xs

```



---

## `:` : an operator symbol starting with a colon is a constructor

\[ [Haskell 2010 Language Report](https://www.haskell.org/onlinereport/haskell2010/haskellch2.html#x7-180002.4) \]



```Haskell

data NonEmpty a = a :| [a]

```



---

## `::` : "type signature"

\[ [Haskell 2010 Language Report](https://www.haskell.org/onlinereport/haskell2010/haskellch4.html#x10-810004.4.1) \]



```Haskell

id :: a -> a

id x =  x

```



---

## `::` : "expression type-signature" (type annotation)

\[ [Haskell 2010 Language Report](https://www.haskell.org/onlinereport/haskell2010/haskellch3.html#x8-560003.16) \]



```Haskell

x = fromIntegral (maxBound::Int)

```



---

## `;` : semicolon in layout rule

\[ [Haskell 2010 Language Report](https://www.haskell.org/onlinereport/haskell2010/haskellch2.html#x7-210002.7) \]



```Haskell

f x = let a = 1; b = 2  

          g y = exp2  

      in exp1 

```



---

## `<-` : lambda-bound in do expression

\[ [Haskell 2010 Language Report](https://www.haskell.org/onlinereport/haskell2010/haskellch3.html#x8-470003.14) \]



```Haskell

f = do

  x <- getLine

  putStrLn x

```



---

## `<-` : "pattern guard"

\[ [Haskell 2010 Language Report](https://www.haskell.org/onlinereport/haskell2010/haskellch4.html#x10-830004.4.3) \]



```Haskell

f x

  | Just y <- g x = 

```



---

## `=>` : context (type class constraint)

\[ [Haskell 2010 Language Report](https://www.haskell.org/onlinereport/haskell2010/haskellch4.html#x10-660004.1.3) \]



```Haskell

subtract :: (Num a) => a -> a -> a

subtract x y = y - x

```



---

## `?` : "ImplicitParams"

\[ [GHC User's Guide](https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/exts/implicit_parameters.html#implicit-parameters) \]



```Haskell

sort :: (?cmp :: a -> a -> Bool) => [a] -> [a]

sort = sortBy ?cmp

```



---

## `@` : "as pattern"

\[ [Haskell 2010 Language Report](https://www.haskell.org/onlinereport/haskell2010/haskellch3.html#x8-580003.17) \]



```Haskell

f s@(x:xs) = 

```



---

## `@` : "type application"

\[ [GHC User's Guide](https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/exts/type_applications.html#visible-type-application) \]



```Haskell

f = read @Int

```



---

## `@` : "type abstraction"

\[ [GHC User's Guide](https://downloads.haskell.org/ghc/latest/docs/users_guide/exts/type_abstractions.html) \]



```Haskell

class C @k a where 

```



---

## `[]` : "empty list" (nil)

\[ [Haskell 2010 Language Report](https://www.haskell.org/onlinereport/haskell2010/haskellch3.html#x8-340003.7) \]

\[ [Haskell 2010 Language Report](https://www.haskell.org/onlinereport/haskell2010/haskellch6.html#x13-1200006.1.3) \]



```Haskell

null [] = True

null _  = False

```



---

## `[ .. ]` : "arithmetic sequence"

\[ [Haskell 2010 Language Report](https://www.haskell.org/onlinereport/haskell2010/haskellch3.html#x8-400003.10) \]



```Haskell

xs = [1..10]

```



---

## `[ | <- ]` : "list comprehension"

\[ [Haskell 2010 Language Report](https://www.haskell.org/onlinereport/haskell2010/haskellch3.html#x8-420003.11) \]



```Haskell

xs = [x^2 | x <- [1..10]] 

```



---

## `[|  |]`, `[e|  |]`, `[d|  |]`, `[t|  |]`, `[p|  |]` : Template Haskell’s quotation syntax (expression, declaration, type, and pattern)

\[ [GHC User's Guide](https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/exts/template_haskell.html#template-haskell) \]



```Haskell

add1 x = [| x + 1 |]

```



---

## `[varid|  |]` : Template Haskell’s quasi-quotation syntax

\[ [GHC User's Guide](https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/exts/template_haskell.html#template-haskell) \]



```Haskell

greet name = [interpolate| Hello, #name! |]

```



---

## `[||  ||]` : Typed Template Haskell’s quotation syntax

\[ [GHC User's Guide](https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/exts/template_haskell.html#template-haskell) \]



```Haskell

add1 x = [|| x + 1 ||]

```



---

## `_` : "wildcard pattern"

\[ [Haskell 2010 Language Report](https://www.haskell.org/onlinereport/haskell2010/haskellch3.html#x8-580003.17) \]



```Haskell

f Red  =

f Blue =

f _    =

```



---

## `_` : unused identifiers beginning with underscore

\[ [GHC User's Guide](https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/using-warnings.html#ghc-flag--Wunused-binds) \]

\[ [Haskell 2010 Language Report](https://www.haskell.org/onlinereport/haskell2010/haskellch2.html#x7-180002.4) \]



```Haskell

_w = True                -- No warning: _w starts with an underscore

```



---

## `_` : "typed hole" (expression level)

\[ [GHC User's Guide](https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/exts/typed_holes.html#typed-holes) \]



```Haskell

sum xs = foldr _ 0 xs

```



---

## `_` : "type wildcard" (type level)

\[ [GHC User's Guide](https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/exts/partial_type_signatures.html#type-wildcards) \]



```Haskell

not' :: Bool -> _

not' x = not x

```



---

## `_` : "extra-constraints wildcard"

\[ [GHC User's Guide](https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/exts/partial_type_signatures.html#extra-constraints-wildcard) \]



```Haskell

arbitCs :: _ => a -> String

```



---

## `_` : "NumericUnderscores"

\[ [GHC User's Guide](https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/exts/numeric_underscores.html#numeric-underscores) \]



```Haskell

million = 1_000_000

```



---

## `\ ->` : "lambda abstraction"

\[ [Haskell 2010 Language Report](https://www.haskell.org/onlinereport/haskell2010/haskellch3.html#x8-260003.3) \]



```Haskell

add1 = \x -> x + 1

```



---

## `\case ->` : "LambdaCase"

\[ [GHC User's Guide](https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/exts/lambda_case.html#lambda-case) \]



```Haskell

f = \case

      Red  -> 2

      Blue -> 1

      _    -> 0

```



---

## `` ` ` `` : "infix notation" - an identifier enclosed in grave accents

\[ [Haskell 2010 Language Report](https://www.haskell.org/onlinereport/haskell2010/haskellch3.html#x8-240003.2) \]



```Haskell

div10 x = x `div` 10

```



---

## `{ }` : brace in layout rule

\[ [Haskell 2010 Language Report](https://www.haskell.org/onlinereport/haskell2010/haskellch2.html#x7-210002.7) \]



```Haskell

f x = case x of {Nothing -> False; Just _ -> True}

```



---

## `{ }` : "record syntax" (datatypes with field labels)

\[ [Haskell 2010 Language Report](https://www.haskell.org/onlinereport/haskell2010/haskellch3.html#x8-490003.15) \]



```Haskell

data MyPoint = Point { x :: Int, y :: Int }

```



---

## `{..}` : "record wildcard"

\[ [GHC User's Guide](https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/exts/record_wildcards.html#record-wildcards) \]



```Haskell

f Vec{..} = 

```



---

## `{-#  #-}` : "compiler pragma"



\[ [Haskell 2010 Language Report](https://www.haskell.org/onlinereport/haskell2010/haskellch12.html#x19-18800012) \]

\[ [GHC User's Guide](https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/exts/pragmas.html) \]

\[ [GHC User's Guide](https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/exts/table.html) \]



```Haskell

{-# LANGUAGE TypeApplications #-}

{-# LANGUAGE GADTs            #-}



{-# INLINE fmap #-}

 ```



---

## `|` : "boolean guard" (guard)

\[ [Haskell 2010 Language Report](https://www.haskell.org/onlinereport/haskell2010/haskellch4.html#x10-830004.4.3) \]



```Haskell

clip255 x

  | x > 255   = 255

  | otherwise = x 

```



---

## `|` : "MultiWayIf"

\[ [GHC User's Guide](https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/exts/multiway_if.html#extension-MultiWayIf) \]



```Haskell

if | x == ":q" -> quit

   | isError x -> errorExit x

   | otherwise -> execCommand x

```



---

## `|` : algebraic datatype declaration

\[ [Haskell 2010 Language Report](https://www.haskell.org/onlinereport/haskell2010/haskellch4.html#x10-690004.2.1) \]



```Haskell

data Maybe a = Nothing | Just a

```



---

## `|` : "functional dependency"

\[ [GHC User's Guide](https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/exts/functional_dependencies.html#functional-dependencies) \]



```Haskell

class Foo a b c | a b -> c where 

```



---

## `~` : "irrefutable pattern"

\[ [Haskell 2010 Language Report](https://www.haskell.org/onlinereport/haskell2010/haskellch3.html#x8-580003.17) \]



```Haskell

f1 ~(as,bs) =

```



## `~` : lazy pattern when Strict is enabled

\[ [GHC User's Guide](https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/exts/strict.html#strict-by-default-pattern-bindings) \]



```Haskell

{-# LANGUAGE Strict #-}



f ~x =

```



---

## `~` : laziness flag

\[ [GHC User's Guide](https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/exts/strict.html#strict-by-default-data-types) \]



```Haskell

data T = C ~a

```



---

## `~` : "equality constraint"

\[ [GHC User's Guide](https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/exts/equality_constraints.html#equality-constraints) \]



```Haskell

class (F a ~ b) => C a b where

```