编写两个函数时出错

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?

Answer 1

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, eg

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