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
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- For more information, see the chapter More on functions
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| infix operators
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a `mappend` b
1+2
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mappend a b
(+) 1 2
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| sections
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(+2)
(3-)
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\x -> x + 2
\x -> 3 - x
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| unary minus[1]
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-x
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negate x
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| tuples[2]
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(x,y)
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(,) x y
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- For more information, see the chapter Haskell/Variables_and_functions
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| function definitions
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f x y = x * y
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f = \x y -> x * y further desugared to f = \x -> \y -> x * y
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| pattern matching
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f [] = 0
f (' ':xs) = f xs
f (x:xs) = 1 + f xs
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f = \l -> case l of
[] -> 0
(' ':xs) -> f xs
(x:xs) -> 1 + f xs
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- 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 []))
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| strings |
"abc" |
['a','b','c'] further desugared to 'a':'b':'c':[] even furtherly desugared to (:) 'a' ((:) 'b' ((:) 'c' []))
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| arithmetic sequences
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[1..5]
[1,3..9]
[1..]
[1,3..]
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enumFromTo 1 5
enumFromThenTo 1 3 9
enumFrom 1
enumFromThen 1 3
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| list comprehensions to functions |
[ x | (x,y) <- foos, x < 2 ]
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let ok (x,y) = if x < 2 then [x] else []
in concatMap ok foos
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| list comprehensions to list monad functions
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[ x | (x,y) <- foos, x < 2 ]
[ (x, bar) | (x,y) <- foos,
x < 2,
bar <- bars,
bar < y ]
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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
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data Ball = Ball
{ x :: Double
, y :: Double
, radius :: Double
, mass :: Double
}
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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_
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| Pattern matching
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getArea Ball {radius = r} = (r**2) * pi
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getArea (Ball _ _ r _) = (r**2) * pi
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| Changing values
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moveBall dx dy ball = ball {x = (x ball)+dx, y = (y ball)+dy}
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moveBall dx dy (Ball x y a m) = Ball (x+dx) (y+dy) a m
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- For more information, see the chapters Understanding monads and do Notation.
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| Sequencing
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do putStrLn "one"
putStrLn "two"
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putStrLn "one" >>
putStrLn "two"
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| Monadic binding
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do x <- getLine
putStrLn $ "You typed: " ++ x
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getLine >>= \x ->
putStrLn $ "You typed: " ++ x
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| Let binding
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do let f xs = xs ++ xs
putStrLn $ f "abc"
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let f xs = xs ++ xs
in putStrLn $ f "abc"
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| Last line
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do x
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x
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| if-then-else
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if x then y else z
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case x of
True -> y
False -> z
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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.
