Help Reading a Hole Error

[Jimmy P]

Hi everyone,

I am trying to write the implementation of flatMap. So far I have:

flatMap ::

  (a -> List b)

  -> List a

  -> List b

flatMap f xs = foldRight undefined undefined _undefined

This gives the following error for the type hole:

Found hole ‘_undefined’ with type: List a0

    Where: ‘a0’ is an ambiguous type variable

    Relevant bindings include

      xs :: List a (bound at src/Course/List.hs:266:11)

      f :: a -> List b (bound at src/Course/List.hs:266:9)

      flatMap :: (a -> List b) -> List a -> List b

        (bound at src/Course/List.hs:266:1)

    In the third argument of ‘foldRight’, namely ‘_undefined’

    In the expression: foldRight undefined undefined _undefined

    In an equation for ‘flatMap’:

        flatMap f xs = foldRight undefined undefined _undefined

 

I know the right peg for this hole is xs, but I dont understand why the haskell compiler has given it a type denoted List a0 and how that relates to the type I'm going to use of List a?

I think it has something to do with the fact that the whole could take a superset of the type List a or something? by why then didn't it just call it List b instead of specifically List a0.

Cheers,

Jim

[Jimmy P]

For those playing at home, the answer is that a0 is just another type variable, it could have just as easily called it List c

Here is a detailed answer on stack overflow http://stackoverflow.com/questions/32344568/how-do-i-read-this-hole-error

[Tony Morris]

The a0 could be anything as long as it isn't a or b.

--
You received this message because you are subscribed to the Google Groups "haskell-exercises" group.
To unsubscribe from this group and stop receiving emails from it, send an email to haskell-exerci...@googlegroups.com.
For more options, visit https://groups.google.com/d/optout.