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.
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(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:
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(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).
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See also
- Currying
- Haskell report: Sections - see for more details