Refactor 'tappend'

[ocramz]

in Data.Align, 'tappend' is clearly partial, moreover it discards some information (the tail of the second Trace). Isn't there a better way to do this?

mt `tappend` (Trace z (t:_)) =
    fmap (\(Trace s ts) -> Trace (s+z) (t:ts)) mt

[robinp]

Hi! If you look at the 3 callsites, it is clear that the second argument is always a singleton list. I don't remember why I abused the second argument into a Trace, this should probably just be a more specific function, getting the two args directly (instead of passing them through a Trace).

While there, it needs typesig, and the fmap should be moved to the callsite. In fact, this can just be a local helper near the callsite. Thanks for bringing up!