Haskell/Syntactic sugar

From Wikibooks, open books for an open world
Jump to: navigation, search

Syntactic sugar is any additional redundant syntax in a programming language that makes the code easier to understand or write.

Summary of the various uses of syntactic sugar in Haskell:

Functions and constructors[edit]

For more information, see the chapter More on functions
description sweet unsweet
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

Lists[edit]

For more information, see the chapters Lists and tuples and More about lists
description sweet unsweet
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)

Records[edit]

Do and proc notation[edit]

For more information, see the chapters Understanding monads and Arrows
description sweet unsweet
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

Other constructs[edit]

description sweet unsweet
if-then-else
if x then y else z
case x of
  True -> y
  False -> z

Literals[edit]

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.

Type level[edit]

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.

Layout[edit]

For more information on layout, see the chapter on Indentation


Notes[edit]

  1. For types in the Num class, including user-defined ones.
  2. Analogous conversions hold for larger tuples.