Techniques for Tracing Constraints

Question

Here\'s the scenario: I\'ve written some code with a type signature and GHC complains could not deduce x ~ y for some x and y. You can usually throw GH

Answer 1

First of all, your function has the wrong type; I am pretty sure it should be (without the context) a -> (a -> b) -> b. GHC 7.10 is somewhat more helpful in pointing that out, because with your original code, it complains about a missing constraint Internal (a -> b) ~ (Internal a -> Internal a). After fixing share's type, GHC 7.10 remains helpful in guiding us:

1. Could not deduce (Internal (a -> b) ~ (Internal a -> Internal b))

2. After adding the above, we get Could not deduce (sup ~ Domain (a -> b))

3. After adding that, we get Could not deduce (Syntactic a), Could not deduce (Syntactic b) and Could not deduce (Syntactic (a -> b))

4. After adding these three, it finally typechecks; so we end up with

   share :: (Let :<: sup,
             Domain a ~ sup,
             Domain b ~ sup,
             Domain (a -> b) ~ sup,
             Internal (a -> b) ~ (Internal a -> Internal b),
             Syntactic a, Syntactic b, Syntactic (a -> b),
             SyntacticN (a -> (a -> b) -> b) fi)
         => a -> (a -> b) -> b
   share = sugarSym Let
   

So I'd say GHC hasn't been useless in leading us.

As for your question about tracing where GHC gets its constraint requirements from, you could try GHC's debugging flags, in particular, -ddump-tc-trace, and then read through the resulting log to see where Internal (a -> b) ~ t and (Internal a -> Internal a) ~ t are added to the Wanted set, but that will be quite a long read.