On 02/04/2015 09:24 PM, Simon Peyton Jones wrote:
Thomas
I was looking at Trac #10045 http://ghc.haskell.org/trac/ghc/ticket/10045. I know exactly what is going on, but my investigation triggered several questions.
I'll have a look too, but first my answers to your questions:
1.What is the state of the ToDos on https://ghc.haskell.org/trac/ghc/wiki/PartialTypeSignatures https://ghc.haskell.org/trac/ghc/wiki/PartialTypeSignatures?
I've updated the TODOs on the wiki page, but I'll summarise the changes: * I've updated the user manual. * I have fixed a TODO in the code, see: http://ghc.haskell.org/trac/ghc/changeset/d108a19cf6cd802c30ff1fa2758dd6aa8c... * You fixed the panic for TODO 1 (see link below), but we still don't get the error messages we (or I would) want when we change the type of the local binding to `_`. http://ghc.haskell.org/trac/ghc/changeset/28299d6827b334f5337bf5931124abc1e5...
2.Is a named wildcard supposed to have any scope? For example:
f :: _a -> b -> _a
f x y = x :: _a
The _a in the signature is not supposed to have any lexical scope over the binding is it? That would be entirely inconsistent with the treatment of ordinary type variables (such as ‘b’ in the example) which only scope if you have an explicit ‘forall b’.
Assuming the answer is “no” (and I really think it should be no), what is the call to tcExtendTyVarEnv2 tvsAndNcs doing in TcBinds.tcRhs? I’m pretty certain it bring into scope only the sig_tvs, and NOT the sig_nwcs.
3.If that is true, I think we may not need the sig_nwcs field of TcSigInfo at all.
Named wildcards follow the scoping behaviour of ScopedTypeVariables but without the forall. See the following example: {-# LANGUAGE PartialTypeSignatures, NamedWildCards #-} {-# OPTIONS_GHC -fno-warn-partial-type-signatures #-} module Scope where f :: _a -> _b -> _a f x y = x :: _b -- Note that this is not your example $ ghc -ddump-types
TYPE SIGNATURES f :: forall w_a w_b. w_a -> w_b -> w_a ...
$ ghc -ddump-types -XScopedTypeVariables
TYPE SIGNATURES f :: forall w_a. w_a -> w_a -> w_a ..
With scoped named wildcards, the second _b (with type _a) must be the same as the first _b and thus _b ~ _a, hence no w_b. That's why there is a call to tcExpandTyVarEnv2 in TcBinds.tcRhs and why we need the sig_nwcs field of TcSigInfo.
4.A TcSigInfo has a sig_id field, which is intended to give the fixed, fully-known polymorphic type of the function. This is used:
·for polymorphic recursion
·as the type of the function to use in the body of the let, even if typechecking the function itself fails.
Neither of these makes sense for partial type sigs. (And in fact, using sig_id for a partial type sig is what gives rise to #10045.) So I’m pretty convinced that we should replace sig_id and sig_partial with a single field sig_id :: Maybe Id, which is Nothing for partial sigs, and (Just ty) for total sigs.
What you say makes sense, but don't we already do something with the same effect (see link)? We only add monomorphic Ids of non-partial type signatures. Or am I missing something? http://git.haskell.org/ghc.git/blob/HEAD:/compiler/typecheck/TcBinds.hs#l130...
I wanted to check with you before blundering in and doing this. Or you could.
RSVP
Thanks
Simon
Cheers, Thomas Disclaimer: http://www.kuleuven.be/cwis/email_disclaimer.htm