> Malcolm Wallace <malcolm.wallace <at> me.com> writes: > > I believe Simon's point is that, if dot is special, we can infer the "Has" type above, even if the compiler is > not currently aware of any actual record types that contain a "foo" field. ... > > (For the record, I deeply dislike making dot special, ...
Simon, Malcolm, here's a solution (at risk of more bikeshedding on syntax). e { foo } * The braces say 'here comes a record'. * Also say 'expect funny business with names'. * The absence of `=` says this is getFld, not update. * This is not currently valid syntax [**], so we don't break code. * It's postfix. (And only a couple more chars than infix dot.) * So perhaps an IDE can see the opening brace and prompt for fields? (Perhaps some IDE's do this already for record update?) * If we need to disambiguate the record type: e :: T Int { foo } -- as the Plan suggests for record update Development would fit into the 'sequence of attack' as 5b., with record update syntax. [**] ghc 7.6.1 rejects the syntax, and suggests you need NamedFieldPuns. But if you set that, you get a weird type error, which suggests ghc is de-sugaring to { foo = foo }. We'd have to fix that. The syntax is valid in a pattern (with NamedFieldPuns). Indeed the proposed syntax echoes pattern match: e .$ (\ (MkFoo { foo }) -> foo ) -- (.$) = flip ($) We'd better insist NamedFieldPuns is on to allow the proposal. Otherwise the syntax would have to be: e { foo = foo } -- ambiguous with update In fact the proposal is an enhancement to NamedFieldPuns, 'repurposed' for OverloadedRecordFields. Possible future development: e { foo, bar, baz } -- produces a tuple ( _, _, _ ) -- with fields in order given -- _not_ 'canonical' order in the data type * By coincidence, that syntax is per one of the dialects for relational algebra projection over a tuple. * Possibly useful for overloaded comprehensions?: [ x { foo, bar, baz } | x <- xs ] [ { foo, bar } | { foo, bar, baz } <- xs, baz = 27 ] AntC _______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users