ekmett/monoids — Issue #2: No Reducer instances for containers
Author: w3rs
Created: 2011-11-22 22:17:39+00:00
State: open

In fresh ghci session:

ghci> :m Data.Set Data.Monoid.Reducer
ghci> unit () :: Set ()

:1:1:
No instance for (Reducer () (Set ()))
arising from a use of `unit'
Possible fix: add an instance declaration for (Reducer () (Set ()))
In the expression: unit () :: Set ()
In an equation for `it': it = unit () :: Set ()
ghci> unit () :: [()] -- for comparison
[()]

Same for Map and probably other containers. I think you should remove these "#ifdef M_CONTAINERS" lines. I searched in google for "M_CONTAINERS", and this seems to be only in monoids package.