| > I've made a ticket and proposal page for removing the monomorphism | > restriction: | > http://hackage.haskell.org/trac/haskell-prime/ticket/131 | > http://hackage.haskell.org/trac/haskell-prime/wiki/NoMonomorphismRestriction | | I think if we do this we really have to do | | http://hackage.haskell.org/trac/haskell-prime/wiki/SpecifyPatternBindingSema... Apropos of the MR, I'd like to invite you to read Let should not be generalised Dimitrios Vytiniotis, Simon Peyton Jones, and Tom Schrijvers Submitted to POPL 2010 http://research.microsoft.com/en-us/um/people/simonpj/papers/constraints/ind... Abstract: From the dawn of time, all derivatives of the classic Hindley-Milner type system have supported implicit generalisation of local let-bindings. Yet, as we will show, for more sophisticated type systems implicit let-generalisation imposes a disproportionate complexity burden. Moreover, it turns out that the feature is very seldom used, so we propose to eliminate it. The payoff is a substantial simplification, both of the specification of the type system, and of its implementation. The paper makes the (somewhat radical) case for not generalising local bindings at all; which would at a stroke remove most of the issues of the MR. (We'd still need to think about the top level.) We'd love to know what any of you think of the idea. Simon | -----Original Message----- | From: haskell-prime-private-bounces@haskell.org [mailto:haskell-prime-private- | bounces@haskell.org] On Behalf Of Simon Marlow | Sent: 27 July 2009 11:13 | To: haskell-prime@haskell.org; haskell-prime-private@haskell.org | Subject: Re: [Haskell'-private] NoMonomorphismRestriction | | On 25/07/2009 16:28, Ian Lynagh wrote: | | > I've made a ticket and proposal page for removing the monomorphism | > restriction: | > http://hackage.haskell.org/trac/haskell-prime/ticket/131 | > http://hackage.haskell.org/trac/haskell-prime/wiki/NoMonomorphismRestriction | | I think if we do this we really have to do | | http://hackage.haskell.org/trac/haskell-prime/wiki/SpecifyPatternBindingSema... | | Which is not strictly speaking a change, but is a necessary | clarification if the MR is removed. I believe the conclusion we came to | in March/April 2008 was to do this. | | Cheers, | Simon | _______________________________________________ | Haskell-prime-private mailing list | Haskell-prime-private@haskell.org | http://www.haskell.org/mailman/listinfo/haskell-prime-private

| . Understanding how to respond to type inference and error messages is | hard enough without having additional differences in innocent-looking | code. Do you think my hope is reasonable that not-generalizing could | lead to better error messages? I don't think it's obvious one way or the other. We'll have to see. | typechecker behaviors to think about. I guess it's still possible to | use explicit type-signatures to make let-bindings polymorphic, Yes, just so | in a way | that is difficult or impossible for lambda or case? (I guess for lambda, | it would require making the lambda into a rank-2 function, though I'm | not sure how to do that syntactically.) Easy: \(x :: forall a. a->a) -> blah Simon

On 6 Aug 2009, at 12:18, Simon Peyton-Jones wrote:

The paper makes the (somewhat radical) case for not generalising local bindings at all; which would at a stroke remove most of the issues of the MR. (We'd still need to think about the top level.)

Only the other day I was writing some code that used a case analysis to build a value I hoped would be polymorphic, and it kept refusing to type-check. Doing the case analysis on a different part of the value, then cutting and pasting the branch code multiple times did however seem to work. Suddenly I remembered about the difference in generalisation between let- and lambda- bindings, and could solve the problem by changing the case to a let! Instantly I could delete the error-prone cut-n-paste copies, and be more assured of the consistency of my code. Regards, Malcolm