On 26 January 2006 16:07, John Hughes wrote:
Simon Marlow wrote:
I wonder if there's an alternative solution along these lines:
- We use ParialTypeSignatures to make bindings monomorphic:
http://haskell.galois.com/cgi-bin/haskell-prime/trac.cgi/wiki/PartialTyp
eSigs
eg.
x :: _ x = (+1)
- we make it a static error for a variable bound by a simple pattern binding ("x = e") to be overloaded, unless a type signature is given. The error message would explain the problem, and how to fix it. Alternatively, we make it a strong warning.
It seems to me that the partial type signatures extension provides a lot of bang for the buck - it gives us a way out of the MR in addition to partial type signatures.
I don't like this. Once students start dropping type signatures (which they do pretty soon for local variables in where-clauses), they would sometimes-- unpredictably as far as they're concerned--get an error message telling them they must put one back in again, but it's enough to write x :: _. Can you imagine explaining to an average student in the first year why they MUST put in a type signature, but it doesn't need to include a type???
Understood, but what about when the student writes '=' instead of ':=' by mistake - most of the time it'll work! But occasionally they fall foul of the reason we had the MR in the first place. Presumably the compiler should emit a noisy warning, but continue anyway? Won't that be confusing? How about an even simpler solution: *All* pattern and variable bindings are monomorphic unless a type signature is given. I wonder how much code this would break? It can't be too bad, because John is suggesting using := for variable bindings everywhere as a starting point. Also, SML users live with something very similar. How often do we really need polymorphism in a variable or pattern binding? I'm guessing probably not that often, because by far the most common use of polymorphism is in functions. You lose if you write mymap = map or mynil = [] and then try to use those at more than one type, but I'm guessing that's quite rare, and you can still give a type signature. I'd be very interested to tweak this in GHC and see how much code still passes the type checker. Cheers, Simon