DuplicateRecordFields + TemplateHaskell

The DuplicateRecordFields extension works by mangling the Names of field selector functions, and treating fields specially in the renamer. Thus if you define

data T = MkT { foo :: Int }

then GHC will actually generate a selector called $sel:foo:MkT, but will resolve foo to refer to this selector.

In general we want this name-mangling to be internal only, but at the moment it is visible when using Template Haskell, because the TH AST doesn't have a way to represent the distinction between the selector name and the field label. For example, if you reify ''T above and then pretty-print it you will currently get

data T = MkT { $sel:foo:MkT :: Int }

which is obviously bad.

I'm not sure how best to address this. We could represent the name as mkNameG_v pkg mod "foo", which will look okay when inspected, but if it is subsequently reified it might be ambiguous. Should we add a new NameFlavour?

Trac metadata

Trac field	Value
Version	7.11
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Template Haskell
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture

changed milestone to %8.0.1

changed weight to 5

added Tbug TemplateHaskell Trac import labels

I've proposed a simple change to how reify works for duplicate record fields, which will mean that normal uses of TH continue to work, but that re-reifying the name of a field will fail.

Trac metadata

Trac field	Value
Differential revisions	- → D1586

assigned to @adamgundry

mentioned in commit 4b161c93

closed

Trac metadata

Trac field	Value
Resolution	Unresolved → ResolvedFixed

added OverloadedRecordFields label

mentioned in issue #14848 (closed)

added Pnormal label

mentioned in issue #21720 (closed)