#16320: Clean up printing of foralls -------------------------------------+------------------------------------- Reporter: goldfire | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.6.3 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Simon and I agreed on several infelicities around the printing of types with foralls. Before cleaning up the code, though, we wanted a ''specification'' of what we should do. Here is what we have. When printing types for users (in GHCi and certain error messages), it is convenient to sometimes suppress `forall`s. Given a type `ty` to print, `forall`s will be suppressed when ''all'' of the following are true: 1. `-fprint-explicit-foralls` is not in effect. 2. The kinds of all bound type variables are Haskell98-style. That is, the kinds consist only of `Type` and `->`. 3. No bound type variable is `Required`. 4. The `forall`s are all at the top. Exception: `forall`s are allowed to be mixed with class constraints, but no `forall`s can appear under a proper `->`. 5. No two quantified type variables are spelled the same. This decision is made once, while looking at the overall type. Once made, it is not challenged: either all `forall`s in a type are printed, or none are. Reasons behind the conditions: 1. No comment. 2. Currently, we print `forall k (a :: k). ...` when there is a bound type variable with a variable kind. But, really, the fact that it's a variable isn't the only way it could be interesting. 3. `Required` type variables must be supplied; omitting these would be very confusing. 4. If a type has nested `forall`s, it's best to print. Reason for exception: the type of `return` is properly `forall (m :: Type -> Type). Monad m => forall (a :: Type). a -> m a`. Note that a `forall` is under a class constraint. But we want to suppress the `forall`s there, too. 5. Imagine this abuse of our language: {{{#!hs class C a b where meth :: forall a. a -> b }}} The type of `meth` is `forall a b. C a b => forall a. a -> b`. This is, actually, a well-formed type, where one type variable shadows another. But if we don't print the `forall`s, any reader will come to the wrong conclusion about the meaning of the type. This is a slight tweak to the current rules, meaning more `forall`s are printed than previously. Pointedly, this will not happen in Haskell98-style code, so they will not appear for beginners. The current state of affairs is messy, on the other hand, where some `forall`s may be suppressed in a type while others are printed, causing more trouble. While in town, any fix should also address comment:17:ticket:11786, which is about a seeming inefficiency of the implementation of `eliminateRuntimeReps`. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16320 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler