Commit c9e4c861 authored by Ryan Scott's avatar Ryan Scott Committed by Ben Gamari
Browse files

Allow visible type application for []

This amounts to a one-line change in `tcExpr`. I've added a Note to
explain what is going on.

This requires a separate change in the pattern-match checker to
account for the fact that typechecked `[]` expressions become
`ConLikeOut`s, not `ExplicitList`s.

Test Plan: make test TEST=T13680

Reviewers: goldfire, mpickering, austin, bgamari

Reviewed By: mpickering, bgamari

Subscribers: rwbarton, thomie, goldfire

GHC Trac Issues: #13680

Differential Revision: https://phabricator.haskell.org/D3733
parent a051b555
......@@ -54,6 +54,7 @@ import NameEnv
import NameSet
import RdrName
import TyCon
import TyCoRep
import Type
import TcEvidence
import VarSet
......@@ -1170,6 +1171,16 @@ tcApp m_herald orig_fun orig_args res_ty
; sel_name <- disambiguateSelector lbl sig_tc_ty
; go (L loc (HsRecFld (Unambiguous lbl sel_name))) args }
-- See Note [Visible type application for the empty list constructor]
go (L loc (ExplicitList _ Nothing [])) [Right ty_arg]
= do { ty_arg' <- tcHsTypeApp ty_arg liftedTypeKind
; let list_ty = TyConApp listTyCon [ty_arg']
; _ <- tcSubTypeDS (OccurrenceOf nilDataConName) GenSigCtxt
list_ty res_ty
; let expr :: LHsExpr GhcTcId
expr = L loc $ ExplicitList ty_arg' Nothing []
; return (idHsWrapper, expr, []) }
go fun args
= do { -- Type-check the function
; (fun1, fun_sigma) <- tcInferFun fun
......@@ -1198,6 +1209,26 @@ mk_app_msg fun = sep [ text "The function" <+> quotes (ppr fun)
mk_op_msg :: LHsExpr GhcRn -> SDoc
mk_op_msg op = text "The operator" <+> quotes (ppr op) <+> text "takes"
{-
Note [Visible type application for the empty list constructor]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Getting the expression [] @Int to typecheck is slightly tricky since [] isn't
an ordinary data constructor. By default, when tcExpr typechecks a list
expression, it wraps the expression in a coercion, which gives it a type to the
effect of p[a]. It isn't until later zonking that the type becomes
forall a. [a], but that's too late for visible type application.
The workaround is to check for empty list expressions that have a visible type
argument in tcApp, and if so, directly typecheck [] @ty data constructor name.
This avoids the intermediate coercion and produces an expression of type [ty],
as one would intuitively expect.
Unfortunately, this workaround isn't terribly robust, since more involved
expressions such as (let in []) @Int won't work. Until a more elegant fix comes
along, however, this at least allows direct type application on [] to work,
which is better than before.
-}
----------------
tcInferFun :: LHsExpr GhcRn -> TcM (LHsExpr GhcTcId, TcSigmaType)
-- Infer type of a function
......
{-# LANGUAGE TypeApplications #-}
module T13680 where
foo :: [Int]
foo = [] @Int
......@@ -561,6 +561,7 @@ test('T13603', normal, compile, [''])
test('T13333', normal, compile, [''])
test('T13585', [extra_files(['T13585.hs', 'T13585a.hs', 'T13585b.hs'])], run_command, ['$MAKE -s --no-print-directory T13585'])
test('T13651', normal, compile, [''])
test('T13680', normal, compile, [''])
test('T13785', normal, compile, [''])
test('T13804', normal, compile, [''])
test('T13822', normal, compile, [''])
......
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