On Tue, Oct 5, 2021 at 12:39 PM David Feuer <david.fe...@gmail.com> wrote:
> To be clear, the proposal to allow different constraints was accepted, but > integrating it into the current, incredibly complex, code was well beyond > the limited abilities of the one person who made an attempt. Totally > severing pattern synonyms from constructor synonyms (giving them separate > namespaces) would be a much simpler design. > It might be simpler in some ways, despite needing yet another syntactic marker, etc. but also would make them a lot less useful for a lot of places where they are used today, e.g. to provide backwards compatibility for old constructors as an API changes, and the like. Before I left MIRI, Cale had started work on these for us. Is that the work you are thinking of, or are you thinking of an earlier effort? -Edward > On Tue, Oct 5, 2021, 12:33 PM Richard Eisenberg <li...@richarde.dev> > wrote: > >> >> >> On Oct 3, 2021, at 5:38 AM, Anthony Clayden <anthony.d.clay...@gmail.com> >> wrote: >> >> > pattern SmartConstr :: Ord a => () => ... >> >> Seems to mean: >> >> * Required constraint is Ord a -- fine, for building >> >> >> Yes. >> >> * Provided constraint is Ord a -- why? for matching/consuming >> >> >> No. Your signature specified that there are no provided constraints: >> that's your (). >> >> >> I'm using `SmartConstr` with some logic inside it to validate/build a >> well-behaved data structure. But this is an ordinary H98 datatype, not a >> GADT. >> >> >> I believe there is no way to have provided constraints in Haskell98. You >> would need either GADTs or higher-rank types. >> >> >> This feels a lot like one of the things that's wrong with 'stupid theta' >> datatype contexts. >> >> >> You're onto something here. Required constraints are very much like the >> stupid theta datatype contexts. But, unlike the stupid thetas, required >> constraints are sometimes useful: they might be needed in order to, say, >> call a function in a view pattern. >> >> For example: >> >> checkLT5AndReturn :: (Ord a, Num a) => a -> (Bool, a) >> checkLT5AndReturn x = (x < 5, x) >> >> pattern LessThan5 :: (Ord a, Num a) => a -> a >> pattern LessThan5 x <- ( checkLT5AndReturn -> (True, x) ) >> >> >> My view pattern requires (Ord a, Num a), and so I must declare these as >> required constraints in the pattern synonym type. Because vanilla data >> constructors never do computation, any required constraints for data >> constructors are always useless. >> >> >> For definiteness, the use case is a underlying non-GADT constructor for a >> BST >> >> > Node :: Tree a -> a -> Tree a -> Tree a >> > >> > pattern SmartNode :: Ord a => () => Tree a -> a -> Tree a -> Tree a >> >> with the usual semantics that the left Tree holds elements less than this >> node. Note it's the same `a` with the same `Ord a` 'all the way down' the >> Tree. >> >> >> Does SmartNode need Ord a to match? Or just to produce a node? It seems >> that Ord a is used only for production, not for matching. This suggests >> that you want a separate smartNode function (not a pattern synonym) and to >> have no constraints on the pattern synonym, which can be unidirectional >> (that is, work only as a pattern, not as an expression). >> >> It has been mooted to allow pattern synonyms to have two types: one when >> used as a pattern and a different one when used as an expression. That >> might work for you here: you want SmartNode to have no constraints as a >> pattern, but an Ord a constraint as an expression. At the time, the design >> with two types was considered too complicated and abandoned. >> >> Does this help? >> >> Richard >> _______________________________________________ >> Glasgow-haskell-users mailing list >> Glasgow-haskell-users@haskell.org >> http://mail.haskell.org/cgi-bin/mailman/listinfo/glasgow-haskell-users >> > _______________________________________________ > Glasgow-haskell-users mailing list > Glasgow-haskell-users@haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/glasgow-haskell-users >
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