haskell - Can someone explain where Applicative instances arise in this code? -

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haskell - Can someone explain where Applicative instances arise in this code? -

isalphanum :: char -> bool isalphanum = (||) <$> isalpha <*> isnum

i can see works, don't understand instances of applicative (or functor) come from.

is mutual go such effort create functions point-free?

this applicative instance ((->) r), functions mutual type. combines functions same first argument type single function duplicating single argument utilize of them. (<$>) function composition, pure const, , here's (<*>) translates to:

s :: (r -> -> b) -> (r -> a) -> r -> b s f g x = f x (g x)

this function perhaps improve known the s combinator.

the ((->) r) functor reader monad, shared argument "environment" value, e.g.:

newtype reader r = reader (r -> a)

i wouldn't it's common sake of making functions point-free, in cases can improve clarity 1 time you're used idiom. illustration gave, instance, can read meaning "is character letter or number".

haskell

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