I like that idea. Do others? Yes, agreed about the difference between ~ and ~~.
~# is unlifted, and is now the type that the solver works on. ~~ is lifted. That's the only difference. But the fact that ~~ is lifted is what allows you to put it in a constraint, because all constraints are lifted. Users should never bother with ~#, but they might with ~~. Richard On Jan 18, 2016, at 12:52 PM, Iavor Diatchki <[email protected]> wrote: > Hello, > > What's the difference between `~~` and `~#` (I assume `~#` is heterogeneous)? > > As for the rest, as far as I understand, `~` is a strict subset of `~~` in > the sense that: > 1. if `a ~ b`, then `a ~~ b` > 2. if `not (a ~ b)`, then `not (a ~~ b)` > 3. if `a ~ b` is a kind error (i.e., the kind of `a` is known to be > different from the kind of `b`), then `not (a ~~ b)` > > So, perhaps it makes sense to have a "smart" pretty printer for `a ~ b`: > > if kindOf a == kindOf b then `a ~ b` else `a ~~ b` > > -Iavor > > > > > > > > On Mon, Jan 18, 2016 at 9:24 AM, Ryan Scott <[email protected]> wrote: > In my ideal world, GHC would remember as much as what the user wrote > as possible in printing error messages. So if the user writes: > > f :: Int ~ Char => ... > > Then GHC would remember that the context was written with a single > tilde, and print out Int ~ Char in the error message explicitly > wherever the full type signature of f is printed. > > What it sounds like, though, is that deep in the guts of the type > inferencer, there's a chance that single-tilde equality might turn > into double-tilde or tilde-hash equality at some point. In those > cases, printing out the particular brand of tilde might get confusing. > In such cases, we might compromise and print out something neutral > like "is equal to". I suppose this would always be the case if you > didn't explicitly write a ~ b and had to infer it. > > I'm not sure about the technical details of this though, i.e., if GHC > actually remembers a ~ b all the way through the > typechecking/inferencing pipeline. > > Ryan S. > _______________________________________________ > ghc-devs mailing list > [email protected] > http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs > > _______________________________________________ > ghc-devs mailing list > [email protected] > http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
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