2009/10/9 wren ng thornton <w...@freegeek.org>: > Cristiano Paris wrote: >> >> On Thu, Oct 8, 2009 at 12:48 PM, Lennart Augustsson wrote: >>> >>> The reason a gets a single type is the monomorphism restriction (read >>> the report). >>> Using NoMonomorphismRestriction your example with a works fine. >> >> Could you explain why, under NoMonomorphismRestriction, this typechecks: >> >> let a = 1 in (a + (1 :: Int),a + (1 :: Float)) >> >> while this not: >> >> foo :: Num a => a -> (Int,Float) >> foo k = (k + (1 :: Int), k + (1.0 :: Float)) > > > Because lambda-binding is always[1] monomorphic, whereas let-binding can be > polymorphic. This distinction is the reason for introducing let-binding in > the first place. If let-bindings weren't allowed to be polymorphic (or if > lambda-bindings were allowed to be) then we could desugar "let x = e in f" > into "(\x -> f) $ e" and simplify the core language.[2]
As it turns out, section 31.6 Why Let and not Lambda of PLAI [*] explains why lambda-bindings are treated monomorphically. [*] http://www.cs.brown.edu/~sk/Publications/Books/ProgLangs/ > Milner's original paper[3] on the topic is still a good introduction to the > field. He couldn't take advantage of subsequent work, so his notation is a > bit outdated (though still understandable). If you're familiar with the > details behind System F, etc. then you should be able to massage the paper > into more modern notation in order to discuss issues like where and how > Rank-2 polymorphism fits in. > > > > [1] That is, under the Hindley--Milner type system. If we add Rank-2 (or > Rank-N) polymorphism then lambdas can bind polymorphic values. > > [2] There's a wrinkle with simplifying the core here. Let-binding is often > introduced in tandem with a special form for declaring recursive values and > mutually recursive groups. Usually this form is simplified somewhat as in > ML's "let rec" or Haskell's invisible laziness recursion. If we remove "let" > then we'll want to use the general version of "rec" and will need to be > explicit about it. > > [3] http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.67.5276 > > -- > Live well, > ~wren > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
