Question

This is the code (Euclid’s algorithm for GCD). Of course there is Prelude.gcd but as an exercise I am implementing my own.

selfGCD :: Integral f => f -> f -> f  
selfGCD a b = if b == 0 then
        return a
    else 
        return (selfGCD a (mod a b))

Using ghci, I get the following error:

two.hs:32:25:  
Couldn't match type `f' with `m0 f'  
  `f' is a rigid type variable bound by  
      the type signature for selfGCD :: Integral f => f -> f -> f  
      at two.hs:31:1  
In the return type of a call of `return'  
In the expression: return a  
In the expression:  
  if b == 0 then return a else return (selfGCD a (mod a b))  

two.hs:34:25:  
Couldn't match type `f' with `m1 f'  
  `f' is a rigid type variable bound by  
      the type signature for selfGCD :: Integral f => f -> f -> f  
      at two.hs:31:1  
In the return type of a call of `return'  
In the expression: return (selfGCD a (mod a b))  
In the expression:  
  if b == 0 then return a else return (selfGCD a (mod a b))  

How can I rectify the problem?

Answer 1

Drop the returns.

In Haskell, return is a function of type

return :: Monad m => a -> m a

and not the return operator you know from imperative languages.

Thus with the returns, the implementation has type

selfGCD :: (Integral a, Monad m) => a -> a -> m a

contrary to the type signature.