Section of an infix operator

In Haskell there is a special syntax for partial application on infix operators. Essentially, you only give one of the arguments to the infix operator, and it represents a function which intuitively takes an argument and puts it on the "missing" side of the infix operator.

• (2^)
  (left section) is equivalent to
  (^) 2
  , or more verbosely
  \x -> 2 ^ x
• (^2)
  (right section) is equivalent to
  flip (^) 2
  , or more verbosely
  \x -> x ^ 2

Like partial application and lambda abstraction, sectioning provides a convenient way of writing some functions without having to explicitly name them:

• (1+)
  (unsugared:
  (+) 1
  ) is the "increment" function,
• (2*)
  is the "double" function,
• ('\t':)
  is the "indent" function,
• (`elem` "AEIOU")
  is the "is-capital-vowel-in-English" function (ignoring the "sometimes Y").

Note: as an exception, the "-" (subtraction) operator cannot do a right section, because that would be interpreted as unary negation in Haskell syntax. The Prelude function "subtract" is provided for this purpose. Instead of , you need to write .

See also

• Currying
• Haskell report: Sections - see for more details