Thanks, so far I’ve just gone through Apoorv’s talk. I can see some reasons this isn’t getting much traction …
- Asking “Was the AMP a good idea?” [Apoorv’s next-to-next-to-last slide]. Do we really want to go through all that library restructuring again?
- “dirty details” [next-to-last slide] I’m getting a fishy smell – as in fishhooks.
- How do we avoid carrying class/constraint dictionaries around the place? (What’s more the
(@)is a methodless dictionary/only there to compile-time verify the code, but it carries constraint dictionaries.) - Because I think the proposed approach is just plain wrong: a large proportion of those dictionaries/constraints are unnecessary; and the 1999 SPJ mail thread I linked above explains why:
pattern-matching on MkT [SPJ thought of] as eliminating a constraint.
But since the dictionary isn’t stored in the constructor we can’t eliminate it.)** I therefore PROPOSE that when pattern matching on a constructor,
** any context on the data type declaration is ignored.
With constructor classes (like instance Functor Set, where output Set b wants Ord b), we in general need an Ord dictionary inside the fmap: whatever morphing fmap is applying, we can’t rely on it preserving the relative ordering of elements. OTOH we don’t want an Ord dictionary for the incoming Set a, because fmap isn’t interested in whatever order the incoming elements are in.
I should mention that unlike in 2010 when DatatypeContexts were taken away, we now have PatternSynonyms that can require constraints for building but not matching (with that double Foo a => Bar a => ... business). So a supplier of a datatype can make the real constructors abstract; export only the synonyms. (I wish Set would do that: it does indeed make the constructors abstract; but provides a bunch of functions (with Ord constraints) for building; most access you have to stream to a List; a PattSyn for lookup within a pattern would be lovely.)
And apologies to @jon if I’m hijacking the thread: I think previous answers concluded the q; if you want more discussion around the o.p. I’ll start a new thread.