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$ ghci
GHCi, version 8.6.3: http://www.haskell.org/ghc/  :? for help
Prelude> import qualified Data.Set as S
Prelude S> S.foo

<interactive>:2:1: error:
    Not in scope: ‘S.foo’
    No module named ‘S’ is imported.

I'm curious why ghci says No module named 'S' is imported. I found it very confusing.

I'd have expected it to say one of three things:

1. Module Data.Set referenced via S does not export foo
2. Constructor S not in scope
3. Variable foo not in scope

The first one would be the most informative, but I can see that the second and the third messages might be quite reasonable as well due to the interpretation of dot as composition. But the message No module S is imported is quite confusing given I just issued import qualified Data.Set as S.

Indeed, if I put this in a file, the error message is crystal clear:

$ cat a.hs
import qualified Data.Set as S

bar = S.foo

$ ghci a.hs
GHCi, version 8.6.3: http://www.haskell.org/ghc/  :? for help
[1 of 1] Compiling Main             ( a.hs, interpreted )

a.hs:3:7: error:
    Not in scope: ‘S.foo’
    Module ‘Data.Set’ does not export ‘foo’.
  |
3 | bar = S.foo
  |       ^^^^^
Failed, no modules loaded.

I'm curious why there's a difference between these two scenarios.