#16320: Clean up printing of foralls -------------------------------------+------------------------------------- Reporter: goldfire | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.6.3 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: | -------------------------------------+------------------------------------- Description changed by bgamari: Old description:
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. Notice that if any foralls are suppressed, then they are all at the "top" of the type (albeit possibly under class constraints).
Reasons behind the conditions:
1. No comment. The ''only'' effect of `-fprint-explicit-foralls` should be to turn off the suppression of foralls, and print all foralls unconditionally.
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. E.g. `forall (a :: Nat). blah`
3. `Required` type variables must be supplied; omitting these would be very confusing. E.b. in `T :: forall k -> k -> Type` it would be jolly confusing to omit the `forall k ->` part, leaving `T :: k -> Type`.
4. If a type has nested `forall`s, it's best to print all the foralls, including any at the top. Example {{{ f :: forall a. (forall b. b -> b) -> a -> a }}} Currently we suppress the outer forall, but with the nested forall we are well out of Haskell98-land and it seems better to display them all.
Reason for exception: the type of `return` is properly {{{ return :: 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, to display {{{ return :: Monad m => a -> m a }}}
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`.
New description: 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. Notice that if any foralls are suppressed, then they are all at the "top" of the type (albeit possibly under class constraints). Reasons behind the conditions: 1. No comment. The ''only'' effect of `-fprint-explicit-foralls` should be to turn off the suppression of foralls, and print all foralls unconditionally. 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. E.g. `forall (a :: Nat). blah` 3. `Required` type variables must be supplied; omitting these would be very confusing. E.b. in `T :: forall k -> k -> Type` it would be jolly confusing to omit the `forall k ->` part, leaving `T :: k -> Type`. 4. If a type has nested `forall`s, it's best to print all the foralls, including any at the top. Example {{{ f :: forall a. (forall b. b -> b) -> a -> a }}} Currently we suppress the outer forall, but with the nested forall we are well out of Haskell98-land and it seems better to display them all. Reason for exception: the type of `return` is properly {{{ return :: 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, to display {{{ return :: Monad m => a -> m a }}} 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#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler