CODEWITHSUNDEEP
haskell
jhegedus
jhegedus

Reputation: 20653

Can the type of head be a->a?

The excerpt below from Gentle Intro to Haskell implies that head's type can be seen as a->a. Is it me who does not understand something or is this a typo?

Probably its just me but then how can head's type be a->a ?

An expression's or function's principal type is the least general type that, intuitively, "contains all instances of the expression". For example, the principal type of head is [a]->a; [b]->a, a->a, or even a are correct types, but too general, whereas something like [Integer]->Integer is too specific.

Upvotes: 5

Views: 131

Answers (1)

daniel gratzer
daniel gratzer

Reputation: 53901

This is a typo, I believe they meant

head :: b -> a

since b is just a random type variable which would unify with [a].

So in order of increasing specificity,

head :: [a] -> a
head :: foo -> a
head :: a

Upvotes: 9

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