Resolving ambiguous instance for multiparam type class

Question

I'm developing a specialized numerical data processing library, and I've come across an error that I can't figure out how to fix. I think it'll be easier to show an example first and then explain my problem. I also apologize for the strange names, I'm having to obfuscate for legal purposes. ``` {-# LANGUAGE MultiParamTypeClasses #-} {-# LANGUAGE FlexibleInstances #-} data MyError = MyError String deriving (Eq, Show) data MyList = MyList [Double] deriving (Eq, Show) data NamedList = NamedList String MyList deriving (Eq, Show) class MyNum a b ret where myAdd :: a -> b -> Either MyError ret myLessThan :: a -> b -> Either MyError Bool instance MyNum MyList Double MyList where myAdd (MyList xs) x = Right $ MyList $ map (+x) xs myLessThan (MyList xs) x = Right $ all (< x) xs instance MyNum NamedList Double NamedList where myAdd (NamedList n l) x = fmap (NamedList n) $ myAdd l x myLessThan (NamedList n l) x = myLessThan l x ``` If I try to compile this, I get the error ``` No instance for (M…

Accepted Answer

You can use functional dependencies to specify that "`ret` is uniquely determined by `a` and `b`".

```
...
{-# LANGUAGE FunctionalDependencies #-}
...
class MyNum a b ret | a b -> ret where
...
```

This lets the typechecker know that it can pick the correct instance definition, knowing only the `a` and `b` from the arguments in your:

```
myLessThan (NamedList n l) x = myLessThan l x
```

The compiler will now complain if you define an additional instance with the same `a` and `b` but a different `ret`, such as

```
instance MyNum MyList Double SomeOtherType where
```

Resolving ambiguous instance for multiparam type class

Solution

Problem