Hi *, I seem to recall this was already suggested in the past, but I can't seem to find it in the archives. For simplicity I'll restate the idea:
foo :: Int -> Int -> (Int,Int)
foo x y = (bar x, bar y)
where
bar x = x+x
results merely in a name-shadowing warning (for -Wall):
foo.hs:4:9: warning: [-Wname-shadowing]
This binding for ‘x’ shadows the existing binding
bound at foo.hs:2:5
However,
import Data.Monoid
(<>) :: String -> String -> String
(<>) = (++)
main :: IO ()
main = putStrLn ("Hi" <> "There")
doesn't allow to shadow (<>), but rather complains about ambiguity:
bar.hs:7:23: error:
Ambiguous occurrence ‘<>’
It could refer to either ‘Data.Monoid.<>’,
imported from ‘Data.Monoid’ at bar.hs:1:1-18
or ‘Main.<>’, defined at bar.hs:4:1
This is of course in line with the Haskell Report, which says in
https://www.haskell.org/onlinereport/haskell2010/haskellch5.html#x11-1010005.3
| The entities exported by a module may be brought into scope in another
| module with an import declaration at the beginning of the module. The
| import declaration names the module to be imported and optionally
| specifies the entities to be imported. A single module may be imported
| by more than one import declaration. Imported names serve as top level
| declarations: they scope over the entire body of the module but may be
| shadowed by *local non-top-level bindings.*
However, why don't we allow this to be relaxed via a new language
extensions, to allow top-level bindings to shadow imported names (and
of course emit a warning)?
Unless I'm missing something, this would help to keep existing and
working code compiling if new versions of libraries start exporting new
symbols (which happen to clash with local top-level defs), rather than
resulting in a fatal name-clash; and have no major downsides.
If this sounds like a good idea, I'll happily promote this into a proper
proposal over at https://github.com/ghc-proposals/ghc-proposals; I
mostly wanted to get early feedback here (and possibly find out if and
where this was proposed before), before investing more time turning
this into a fully fledged GHC proposal.
Cheers,
HVR
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