[Haskell-cafe] SYB supports genuine gmap

oleg at okmij.org oleg at okmij.org
Fri Jan 3 05:21:22 UTC 2014

For the record, I have described the implementation of generic map
(which is fmap implemented generically, once and for all).


The article stresses a drawback of a fake gmap in terms of gfoldl: a
fake gmap is non-parametric. It fails the law gmap f . gmap g == gmap
(f.g). OTH, the genuine gmap satisfies the law, as behooves to the
generic version of fmap. Although gmap is implemented in terms of the
parametricity-breaking type-introspecting SYB, the gmap is parametric

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