Difference between revisions of "Section of an infix operator"

From HaskellWiki
Jump to navigation Jump to search
(description of the concept)
 
(2 intermediate revisions by one other user not shown)
Line 1: Line 1:
−
In Haskell there is a special syntax for [[partial application]] on [[infix operator]]s.
 +
In Haskell there is a special syntax for [[partial application]] on [[infix operator]]s. 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.
   
−
* <hask>(2^)</hask> is equivalent to <hask>(^) 2</hask>
 +
* <hask>(2^)</hask> (left section) is equivalent to <hask>(^) 2</hask>, or more verbosely <hask>\x -> 2 ^ x</hask>
−
* <hask>(^2)</hask> is equivalent to <hask>flip (^) 2</hask>
 +
* <hask>(^2)</hask> (right section) is equivalent to <hask>flip (^) 2</hask>, or more verbosely <hask>\x -> x ^ 2</hask>
   
   
  +
Like [[partial application]] and [[lambda abstraction]], sectioning provides a convenient way of writing some functions without having to explicitly name them:
  +
* <hask>(1+)</hask> (unsugared: <hask>(+) 1</hask>) is the "increment" function,
  +
* <hask>(2*)</hask> is the "double" function,
  +
* <hask>('\t':)</hask> is the "indent" function,
  +
* <hask>(`elem` "AEIOU")</hask> 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 <hask>(- e)</hask>, you need to write <hask>(subtract e)</hask>.
  +
  +
== See also ==
  +
  +
* [[Currying]]
  +
* [http://www.haskell.org/onlinereport/exps.html#sections Haskell report: Sections] - see for more details
  +
  +
[[Category:Glossary]]
 
[[Category:Syntax]]
  
[[Category:Syntax]]

Revision as of 21:40, 1 February 2009

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 (- e), you need to write (subtract e).

See also

Retrieved from "https://wiki.haskell.org/index.php?title=Section_of_an_infix_operator&oldid=26232"