CODEWITHSUNDEEP
haskelltype-inference
schoettl
schoettl

Reputation: 360

Make function with (Maybe a) parameter "generic"

let updateFunc = updatedMaybeProperty srcTitle targetTitle :: (Title -> Maybe a) -> Maybe a
    _ = updateFunc (titleVersion      :: Title -> Maybe Text)
    _ = updateFunc (titleYearProduced :: Title -> Maybe Integer)

I get this error in line 3:

    • Couldn't match type ‘Text’ with ‘Integer’
      Expected type: Title -> Maybe Text
        Actual type: Title -> Maybe Integer
    • In the first argument of ‘updateFunc’, namely
        ‘(titleYearProduced :: Title -> Maybe Integer)’

Apparently, in line 2, the compiler infers the type for Maybe a and decides a must always be Text.

How can I prevent this and make updateFunc "generic" so that it works with different types for a?

Upvotes: 0

Views: 82

Answers (1)

chi
chi

Reputation: 116139

Try annotating the binding, not the expression.

let updateFunc :: (Title -> Maybe a) -> Maybe a
    updateFunc = updatedMaybeProperty srcTitle targetTitle
    _ = updateFunc (titleVersion      :: Title -> Maybe Text)
    _ = updateFunc (titleYearProduced :: Title -> Maybe Integer)

Upvotes: 6

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