Syntactic sugar refers to any redundant type of syntax in a programming language that is redundant to the main syntax but which (hopefully) makes the code easier to understand or write.
Functions and constructors [ edit | edit source ]
For more information, see the chapter More on functions
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infix operators
a `mappend` b
1+2
mappend a b
(+) 1 2
sections
(+2)
(3-)
\x -> x + 2
\x -> 3 - x
unary minus^{[1]}
-x
negate x
tuples^{[2]}
(x,y)
(,) x y
For more information, see the chapter Haskell/Variables_and_functions
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function definitions
f x y = x * y
f = \x y -> x * y further desugared to f = \x -> \y -> x * y
pattern matching
f [] = 0
f (' ':xs) = f xs
f (x:xs) = 1 + f xs
f = \l -> case l of
[] -> 0
(' ':xs) -> f xs
(x:xs) -> 1 + f xs
For more information, see the chapters Lists and tuples , Lists II , Lists III , Understanding monads/List and MonadPlus
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lists
[1,2,3]
1:2:3:[] further desugared to (:) 1 ((:) 2 ((:) 3 []))
strings
"abc"
['a','b','c'] further desugared to 'a':'b':'c':[] even furtherly desugared to (:) 'a' ((:) 'b' ((:) 'c' []))
arithmetic sequences
[1..5]
[1,3..9]
[1..]
[1,3..]
enumFromTo 1 5
enumFromThenTo 1 3 9
enumFrom 1
enumFromThen 1 3
list comprehensions to functions
[ x | (x,y) <- foos, x < 2 ]
let ok (x,y) = if x < 2 then [x] else []
in concatMap ok foos
list comprehensions to list monad functions
[ x | (x,y) <- foos, x < 2 ]
[ (x, bar) | (x,y) <- foos,
x < 2,
bar <- bars,
bar < y ]
foos >>= \(x, y) ->
guard (x < 2) >>
return x
foos >>= \(x, y) -> guard (x < 2) >>
bars >>= \bar ->
guard (bar < y) >>
return (x, bar)
-- or equivalently
do (x, y) <- foos
guard (x < 2)
bar <- bars
guard (bar < y)
return (x, bar)
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Creation
data Ball = Ball
{ x :: Double
, y :: Double
, radius :: Double
, mass :: Double
}
data Ball = Ball
Double
Double
Double
Double
x :: Ball -> Double
x (Ball x_ _ _ _) = x_
y :: Ball -> Double
y (Ball _ y_ _ _) = y_
radius :: Ball -> Double
radius (Ball _ _ radius_ _) = radius_
mass :: Ball -> Double
mass (Ball _ _ _ mass_) = mass_
Pattern matching
getArea Ball {radius = r} = (r**2) * pi
getArea (Ball _ _ r _) = (r**2) * pi
Changing values
moveBall dx dy ball = ball {x = (x ball)+dx, y = (y ball)+dy}
moveBall dx dy (Ball x y a m) = Ball (x+dx) (y+dy) a m
For more information, see the chapters Understanding monads and do Notation .
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Sequencing
do putStrLn "one"
putStrLn "two"
putStrLn "one" >>
putStrLn "two"
Monadic binding
do x <- getLine
putStrLn $ "You typed: " ++ x
getLine >>= \x ->
putStrLn $ "You typed: " ++ x
Let binding
do let f xs = xs ++ xs
putStrLn $ f "abc"
let f xs = xs ++ xs
in putStrLn $ f "abc"
Last line
do x
x
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if-then-else
if x then y else z
case x of
True -> y
False -> z
A number (such as 5) in Haskell code is interpreted as fromInteger 5
, where the 5
is an Integer
. This allows the literal to be interpreted as Integer
, Int
, Float
etc. Same goes with floating point numbers such as 3.3
, which are interpreted as fromRational 3.3
, where 3.3
is a Rational
. GHC has OverloadedStrings
extension, which enables the same behaviour for string types such as String
and ByteString
varieties from the Data.ByteString
modules.
The type [Int]
is equivalent to [] Int
. This makes it obvious it is an application of []
type constructor (kind * -> *
) to Int
(kind *
).
Analogously, (Bool, String)
is equivalent to (,) Bool String
, and the same goes with larger tuples.
Function types have the same type of sugar: Int -> Bool
can also be written as (->) Int Bool
.
For more information on layout, see the chapter on Indentation
Notes
↑ For types in the Num class, including user-defined ones.
↑ Analogous conversions hold for larger tuples.