#14662: Partial type signatures + mutual recursion = confusion -------------------------------------+------------------------------------- Reporter: goldfire | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj):
If the user quantifies a partial type signature (either by using forall, or just using an out-of-scope type variable and using Haskell's implicit quantification), why can't we use polymorphic recursion with that variable?
I recommend caution. Partial type signatures were implemented fairly quickly by a PhD student some time ago, but I have spent many hours since slowly fixing bugs in the implementation. It's much trickier than I supposed at first. (So I'm not blaming him! And its a very nice feature.) Here's a core principle: partial type signatures go entirely via the `InferGen` plan: that is, we use only monomorphic rcursion exactly as if there was no signature. All the signature does is to impose a partial shape on the type of the RHS, including restricting some parts of that type to type variables -- hence the `SigTvs`. (I don't share your dislike; `SigTvs` are very nice actually.) Yes, you could imagine some kind of partial polymorphic recursion, but I don't think it makes sense. Eg {{{ f :: forall a. _ -> a f = ...f @Int...f @Bool..... }}} That wildcard might end up being `a`! If it did, then would the recursive occurrences get instantiate to `Int->Int` and `Bool->Bool`. Certainly not. This way lies madness. The current implementation is quite complicated enough. I agree that documentation is lacking: no, polymorphic recursion is absolutely not available for functions with a partial type signature. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14662#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler