I have a file with the following relevant lines:

import qualified Control.Concurrent as C

...

isCancelled = C.isEmptyMvar . stopConcurrentProcess

Trying to compile this or run it in ghci yields a puzzling error message:

Not in scope: `C.isEmptyMvar'

Perhaps you meant one of these:

`C.isEmptyMVar' (imported from Control.Concurrent),

`C.isEmptyChan' (imported from Control.Concurrent)

Any idea what could be going wrong?