Commit 2f8cd14f authored by Simon Peyton Jones's avatar Simon Peyton Jones

Narrow the use of record wildcards slightly

In reviewing the fix to Trac #12130 I found the wild-card
fill-in code for ".." notation in record constructions hard
to understand.  It went to great contortions (including the
find_tycon code) to allow
    data T = C { x, y :: Int }
    f x = C { .. }
to expand to
    f x = C { x = x, y = y }
where 'y' is an /imported function/!  That seems way over the top
for what record wildcards are supposed to do.

So I have narrowed the record-wildcard expansion to include only
/locally-bound/ variables; i.e. not top level, and certainly not
imported.

I don't think anyone is using record wildcards in this bizarre way, so
I don't expect any fallout. Even if there is, you can easily
initialise fields with eponymous but imported values by hand.

An intermediate position would be to allow /local/ top-level
definitions.  But I doubt anyone is doing that either.

Let's see if there's any fallout.  It's a local change, easy to
revert, so I've just gone ahead to save everyone's time.
parent 643706e4
......@@ -588,23 +588,13 @@ rnHsRecFields ctxt mk_arg (HsRecFields { rec_flds = flds, rec_dotdot = dotdot })
; con_fields <- lookupConstructorFields con
; when (null con_fields) (addErr (badDotDotCon con))
; let present_flds = map (occNameFS . rdrNameOcc) $ getFieldLbls flds
parent_tc = find_tycon rdr_env con
-- For constructor uses (but not patterns)
-- the arg should be in scope (unqualified)
-- ignoring the record field itself
-- the arg should be in scope locally;
-- i.e. not top level or imported
-- Eg. data R = R { x,y :: Int }
-- f x = R { .. } -- Should expand to R {x=x}, not R{x=x,y=y}
arg_in_scope lbl
= rdr `elemLocalRdrEnv` lcl_env
|| notNull [ gre | gre <- lookupGRE_RdrName rdr rdr_env
, case gre_par gre of
ParentIs p -> Just p /= parent_tc
FldParent p _ -> Just p /= parent_tc
PatternSynonym -> False
NoParent -> True ]
where
rdr = mkVarUnqual lbl
arg_in_scope lbl = mkVarUnqual lbl `elemLocalRdrEnv` lcl_env
dot_dot_gres = [ (lbl, sel, head gres)
| fl <- con_fields
......@@ -646,11 +636,12 @@ rnHsRecFields ctxt mk_arg (HsRecFields { rec_flds = flds, rec_dotdot = dotdot })
| Just gre <- lookupGRE_Name env con_name
= case gre_par gre of
ParentIs p -> Just p
_ -> Nothing
_ -> Nothing -- Can happen if the con_name
-- is for a pattern synonym
| otherwise = Nothing
-- This can happen if the datacon is not in scope
-- and we are in a TH splice (Trac #12130)
-- Data constructor not lexically in scope at all
-- See Note [Disambiguation and Template Haskell]
dup_flds :: [[RdrName]]
-- Each list represents a RdrName that occurred more than once
......@@ -659,6 +650,22 @@ rnHsRecFields ctxt mk_arg (HsRecFields { rec_flds = flds, rec_dotdot = dotdot })
(_, dup_flds) = removeDups compare (getFieldLbls flds)
{- Note [Disambiguation and Template Haskell]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider (Trac #12130)
module Foo where
import M
b = $(funny)
module M(funny) where
data T = MkT { x :: Int }
funny :: Q Exp
funny = [| MkT { x = 3 } |]
When we splice, neither T nor MkT are lexically in scope, so find_tycon will
fail. But there is no need for diambiguation anyway, so we just return Nothing
-}
rnHsRecUpdFields
:: [LHsRecUpdField RdrName]
-> RnM ([LHsRecUpdField Name], FreeVars)
......
......@@ -3053,8 +3053,10 @@ More details:
unqualified).
- In the case of expressions (but not patterns), the variable ``f``
is in scope unqualified, apart from the binding of the record
selector itself.
is in scope unqualified, and is not imported or bound at top level.
For example, ``f`` can be bound by an enclosing pattern match or
let/where-binding. (The motivation here is that it should be
easy for the reader to figure out what the "``..``" expands to.)
These rules restrict record wildcards to the situations in which the
user could have written the expanded version. For example ::
......
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