Error while composing two functions

Question

I try to compose two function with type specifying.

foo :: Num a => a -> a
foo a = a + 2

bar :: Num a => a -> a
bar a = a * 2

fooBarCompose :: (Num a, Num b, Num c) => (a -> b) -> (c -> a) -> c -> b
fooBarCompose f g = f . g

My module compiles, but in runtime when I invoke

fooBarCompose bar foo

I get an error:

No instance for (Show (b0 -> b0))
  (maybe you haven't applied enough arguments to a function?)
  arising from a use of ‘print’
In the first argument of ‘print’, namely ‘it’
In a stmt of an interactive GHCi command: print it

Any ideas why I get this?

leftaroundabout · Accepted Answer · 2016-03-13 14:34:28Z

11

Any ideas why I get this?

You don't. Everything you've written works just fine. You can use fooBarCompose bar foo in any program you want.

Only, if you try to evaluate it in GHCi, it has a problem: fooBarCompose bar foo is a function. How the heck is it supposed to show a function? Display an exhaustive list of all possible inputs and corresponding results? Clearly not feasible. GHCi uses print under the hood, which simply invokes show. And, well, because it's not possible to show a function, it gives you an error message saying exactly this.

OTOH, the result of applying a function to any single value can easily be shown, e.g.

> fooBarCompose bar foo 2  -- aka `bar . foo $ 2`
8

answered Mar 13, 2016 at 14:34

leftaroundabout

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