Introduce shortcut for "on (==)" much like there's a shortcut for "on compare".
A common use case for on (from Data.Function) is to use it with compare, e.g. compare on snd. In fact, this pattern is so common that there's a convenient comparing function which provides a shortcut for this use case such that one can write
sortBy (comparing snd)instead of
sortBy (compare `on` snd)I think another common use case is to use on together with (==) as in
groupBy ((==) `on` snd)In a similiar vein as with comparing, I think it would be nice if there was a function which encapsulates this use case, like
equating :: Eq b => (a -> b) -> a -> a -> Bool
equating = on (==)such that one can write
groupBy (equating snd)The (IMHO fairly nice) name equating is not my idea but was suggested by //ClaudiusMaximus// of #haskell fame.
Trac metadata
| Trac field | Value |
|---|---|
| Version | 7.8.2 |
| Type | FeatureRequest |
| TypeOfFailure | OtherFailure |
| Priority | normal |
| Resolution | Unresolved |
| Component | libraries/base |
| Test case | |
| Differential revisions | |
| BlockedBy | |
| Related | |
| Blocking | |
| CC | ekmett, hvr |
| Operating system | |
| Architecture |