State a law for foldMap
After being prodded by /u/random_crank on reddit, I realized there is a law we can state for foldMap:
A monoid homomorphism g is a function such that
g mempty = mempty
g (mappend a b) = mappend (g a) (g b)holds.
For any monoid homomorphism g:
foldMap (g . f) = g . foldMap fThis isn't strictly a free theorem as it relies on the Monoid laws, but it is true and has the same general flavor as the equally complicated free theorem for foldr.
This law is a bit technical, but is a nice tool for equational reasoning, and gives rise to a much nicer foldMap-based version of the "banana split" theorem for foldr.
foldMap f &&& foldMap g = foldMap (f &&& g)Trac metadata
| Trac field | Value |
|---|---|
| Version | 7.10.1-rc1 |
| Type | FeatureRequest |
| TypeOfFailure | OtherFailure |
| Priority | low |
| Resolution | Unresolved |
| Component | Core Libraries |
| Test case | |
| Differential revisions | |
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| Blocking | |
| CC | core-libraries-committee@haskell.org |
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