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Section of an infix operator

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In Haskell there is a special syntax for [[partial application]] on [[infix operator]]s.
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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>
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* <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>
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* <hask>(^2)</hask> (right section) is equivalent to <hask>flip (^) 2</hask>, or more verbosely <hask>\x -> x ^ 2</hask>
   
   
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Like [[partial application]] and [[lambda abstraction]], sectioning provides a convenient way of writing some functions without having to explicitly name them:
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* <hask>(1+)</hask> (unsugared: <hask>(+) 1</hask>) is the "increment" function,
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* <hask>(2*)</hask> is the "double" function,
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* <hask>('\t':)</hask> is the "indent" function,
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* <hask>(`elem` "AEIOU")</hask> is the "is-capital-vowel-in-English" function (ignoring the "sometimes Y").
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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>.
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== See also ==
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* [[Currying]]
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* [http://www.haskell.org/onlinereport/exps.html#sections Haskell report: Sections] - see for more details
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[[Category:Glossary]]
 
[[Category:Syntax]]
 
[[Category:Syntax]]

Latest 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)
.

[edit] See also