[Haskell-cafe] Re-order type

[Edward Z. Yang]

Excerpts from André Batista Martins's message of Sat Oct 09 17:45:05 -0400 2010:
> Hello,
>  exists any algorithm  to determine how terms can be changed to safisty the
> type of one function?

Your terminology is a little off: in particular, your example doesn't include
any "terms" (which are understood to be the things to the right of the =,
not the double-colon).

The term you are actually looking for is "unification".  One way of looking
at this is that you have two types that are the same type, but due to their
structure they impose various constraints on each other. Solve all of the
constraints and you have a unified type (indeed, the "best" such type in
many type systems--the correct term is principal type), and if you can't
solve them then you have a type error.

Cheers,
Edward