Question

I have a function which contains an abstract data type as a parameter. In order for me to be able to equate this abstract data type I used:

myfunction:: Eq x=> [x]->[x]->[x]

So it takes in two lists of x, and outputs a list of [x]

However, when I call it from another function:

anotherfunction::[x] -> [x] -> [x]
anotherfunction a b = myfunction a b

it says

No instance for (Eq x) arising from a use of myfunction, in the
expression myfunction a b

However, if I call myfunction from the console, with the two arguments it works fine.

How do I solve this?

Answer 1

Use Eq in the type of anotherfunction.

anotherfunction::Eq x => [x] -> [x] -> [x]
anotherfunction a b = myfunction a b

myfunction works only with types that are in the Eq class. But your current definition of anotherfunction claims it can work with any type. This would cause a problem if you called anotherfunction with a type that is not in the Eq class – it wouldn’t be able to call myfunction.