Illegal type error when using Integer as argument

Question

Beginner trying to do the 99 problems. Here is my attempt to solve the third problem (yes):

elementAt :: [a] -> Int -> Maybe a
elementAt _ x | x <= 0 = Nothing
elementAt [] x | x > 0 = Nothing
elementAt (x: _) 1 = Just x
elementAt (_: xs) x | x > 1 = elementAt xs (x-1)

testElementAt :: IO ()
testElementAt = do
    print (elementAt []::[Int] 5)
    print (elementAt []::[Int] 0)
    print (elementAt [1, 2, 3] 2)
    print (elementAt [1, 2, 3] 5)
    print (elementAt [1, 2, 3] 1)
    print (elementAt [1, 2, 3] 0)

main :: IO ()
main = do
    testElementAt

Error message:

error:
    Illegal type: ‘5’ Perhaps you intended to use DataKinds
        print (elementAt []::[Int] 5)
                                   ^

I guess it has something to do with 5 being able to be Int as well as Float? (Just like [] which I have to type it with ::[Int] to pass the compiler?) However, the same trick does not seem to work.

What should I do?

Without parentheses, elementAt []::[Int] 5 is parsed as elementAt []::([Int] 5). — chi
– chi, Commented Jan 22, 2022 at 21:37

willeM_ Van Onsem · Accepted Answer · 2022-01-22 20:21:04Z

3

5 is here part of the type signature. If you want to specify the type of the list, you do that with:

print (elementAt ([] :: [Int]) 5)

here we thus give a type hint that the empty list is a list of Ints. The 5 in this case is thus seen as the second parameter.

answered Jan 22, 2022 at 20:21

willeM_ Van Onsem

482k33 gold badges483 silver badges624 bronze badges

Sign up to request clarification or add additional context in comments.

5 Comments

Clément Dato Over a year ago

Thank you. But I tried this. But without the space. Now it worked. :)) (I heard about the space-sensitivity, but not expected to be this sensitive :))

willeM_ Van Onsem Over a year ago

@ClémentDato: what do you mean with "without the space"?

Clément Dato Over a year ago

I did it like print (elementAt ([]::[Int]) 5)

willeM_ Van Onsem Over a year ago

@ClémentDato: well the spaces before and after the :: are optional, so that does not matter. What matters is that you use parenthesis to make sure the type signature is applied to [], and that 5 is not seen as a type parameter.

Clément Dato Over a year ago

Thank you. You are correct. I can not replicate the no-space error any more. Maybe it's just some of my unintended beginner side effect (forgetting to save before compiling).

Collectives™ on Stack Overflow

Illegal type error when using Integer as argument

1 Answer 1

5 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

5 Comments

Your Answer

Sign up or log in

Post as a guest

Related