#14046: “Illegal type synonym family application in instance” is too strict in the presence of functional dependencies -------------------------------------+------------------------------------- Reporter: lexi.lambda | Owner: (none) Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: #3485 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by AntC):
..., I’m still not completely sure why this wouldn’t be a reasonable change.
Try to figure out the rules: * The type family application cannot appear in a 'argument position' of a FunDep. * There might be multiple FunDeps/multi-directional. (An instance parameter might be in 'argument position' by one FunDep, but 'result position' from another.) * The variables in a type family application must be already bound by parameters appearing in 'argument position'. (Or possibly determined from those via constraints -- essentially the Liberal Coverage Conditions.) It's surely easier to have a blanket rule 'no type families'/achieve the effect with equality constraints.
The coverage condition itself is precedent for weakening of restrictions in the presence of fundeps (in that case, instance decidability), and this does not seem meaningfully different to me.
Even `UndecidableInstances` doesn't allow complete chaos. If that's what you want, there's a feature request somewhere dubbed 'dysfunctional functional dependencies'.
I could be convinced by an example of the sort that SPJ alluded to, ...
First consider type synonyms appearing in instance heads: {{{ {-# LANGUAGE FlexibleInstances #-} instance Multi String String where ... -- immediately expanded to instance Multi [Char] [Char] where ... -- not: instance (b0 ~ [Char]) => Multi [Char] b0 }}} OK Let's try to concoct an example per SPJ. Suppose {{{ type family Fam a where Fam a = Int instance Multi (Maybe a) (Fam a) where ... -- immediately expanded to instance Multi (Maybe a) Int where ... }}} At the use site, GHC will wait until `b` is resolved to `Int` before it will use the instance.
... in which an instance of the form Multi X (Y a) behaves differently from (b ~ Y a) => Multi X b, but I’m not sure sure what such an example would look like or how it would differ from a user’s point of view.
You mean `Y` is a type family? I'm not seeing that `instance Multi X (Y a)` would ever be a good idea: there's nothing to tell what `a` is. Even if the use site tells `Y a ~ Int`, that's no help (unless `Y` is injective -- but that's a different scenario). -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14046#comment:7 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler