Difference between revisions of "Section of an infix operator"
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(sectioning examples taken from article Currying) |
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− | 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> |
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* <hask>('\t':)</hask> is the "indent" function, |
* <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"). |
* <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 == |
== See also == |
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* [[Currying]] |
* [[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:Glossary]] |
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 toflip (^) 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
- Currying
- Haskell report: Sections - see for more details