One thing that often comes up in these discussions is using types to manage units of measure. I think this is an awesome example, so here's a sample of my favorite library for it (unittyped)[1]:

    *Main> (1 meter / second) * 5 second
    5.0 m/s⋅s
    *Main> 2 meter + (1 meter / second) * 5 second
    7.0 m

One really cute trick is that the various si prefixes are actually functions. So instead of defining every combination of milli-, centi-, deci and so on, you can write them like this:

    Main*> 1 kilo meter + 324 centi meter
    1.00324 km

The library can convert between compatible units automatically, so you never have to worry about the conversion factor:

    Main*> 1.23 kilo meter `as` mile
    0.7642865664519207 mile

Since the prefixes are systematic, you can come up with your own bastardized units:

    Main*> 1 centi mile `as` kilo inch
    0.6336000000000002 kin

Of course, if we try to combine incompatible units, we get a type error. And this shows the most glaring problem with this library :P.

    Main*> 10 second + 2 inch

    <interactive>:38:11:
        Couldn't match type 'GHC.Types.False with 'GHC.Types.True
        When using functional dependencies to combine
          UnitTyped.And 'GHC.Types.False 'GHC.Types.False 'GHC.Types.False,
            arising from the dependency `a b -> c'
            in the instance declaration in `UnitTyped'
          UnitTyped.And 'GHC.Types.False 'GHC.Types.False 'GHC.Types.True,
            arising from a use of `+' at <interactive>:38:11
        In the expression: 10 second + 2 inch
        In an equation for `it': it = 10 second + 2 inch

This message is absolutely hideous. But it's still better than nothing. I think some mechanism for letting libraries customize type error messages would be awesome, but I don't know what that would look like.

[1]: https://bitbucket.org/xnyhps/haskell-unittyped/wiki/Examples