According to my guess:

shouldThrow receives `IO a` as a first argument.
So, both ` myButLast [] ` and ` myButLast [1] ` must be some `IO a`
Then, ` myButLast` 's type is inferred as `[IO a] -> IO a` when given `[]`: the element of the list is `IO a` (Which actually would be `IO ()` by the default rule).
Similarly, ` myButLast [1] ` 's `a` (the type of the element of the list) must be `IO something`. But any of `IO something` can't be a `Num`'s instance (as the error message " No instance for (Num (IO a0)) arising from the literal ‘1’ " says).

That's why ` myButLast [1] ` can't be compiled.

To fix it, use `evaluate`:

`evaluate myButLast [1]`

One more note: maybe you know we usually should not use `error`, which raises an error in a pure function.
So, we usually handle errors in an `IO` context. That's why `shouldThrow` receives an `IO a`.

2018年7月9日(月) 2:57 Paweł Bałaga <ppbalaga@gmail.com>:

Hello fellow Haskellers!

I'm approaching learning the basics of Haskell by going through https://wiki.haskell.org/99_questions. At the same time I write tests for my code in Hspec. Consider question no. 2: "Find the last but one element of a list".

My solution:
-- Problem 02
myButLast :: [a] -> a
myButLast [] = error "Empty list"
myButLast [x] = error "List has only one element"
myButLast [x1,x2] = x1
myButLast (x:xs) = myButLast xs

and a a test:
describe "02 myButLast" $ do
it "returns the last but one element of a list" $ do
myButLast [] `shouldThrow` anyErrorCall
myButLast [1] `shouldThrow` anyErrorCall -- <- this line causes the problem
myButLast [1..4] `shouldBe` 3
myButLast ['x','y','z'] `shouldBe` 'y'
myButLast "abc" `shouldBe` 'b'

Building tests with stack test command causes the below compilation error:

    • No instance for (Num (IO a0)) arising from the literal ‘1’
    • In the expression: 1
      In the first argument of ‘myButLast’, namely ‘[1]’
      In the first argument of ‘shouldThrow’, namely ‘myButLast [1]’
   |
27 |             myButLast [1] `shouldThrow` anyErrorCall
   |

From what I understand, type of myButLast [1] is different than expected by shouldThrow. What I don't understand is why exactly it behaves so and how to fix this problem. Only that one assertion doesn't compile. The others are fine. Particularly, why does myButLast [] `shouldThrow` anyErrorCall work but with one element it doesn't?

Can you please give me a hand?

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