In particular, if fmap <http://hackage.haskell.org/package/base-4.10.1.0/docs/Data-Functor.html#v:fmap> is an expensive operation, it is likely better to use liftA2 <http://hackage.haskell.org/package/base-4.10.1.0/docs/Control-Applicative.html#v:liftA2> than to (fmap <http://hackage.haskell.org/package/base-4.10.1.0/docs/Data-Functor.html#v:fmap> over the structure *and then use <*> <http://hackage.haskell.org/package/base-4.10.1.0/docs/Control-Applicative.html#v:-60--42--62->)* . (emphasis and parens added).

Consider any data constructor that has a strict spine and look at f <$> m <*> n This has to walk `m` twice. Once for the fmap on the first argument, and once for the <*>, but the liftA2 version can walk `m` once fusing things together. -Edward On Tue, Jan 30, 2018 at 4:42 AM, Damien BIHEL <biheldamien@gmail.com> wrote:

In the documentation here <http://hackage.haskell.org/package/base-4.10.1.0/docs/Control-Applicative.html#v:liftA2> it's written that for some functors liftA2 has better performance than fmap. Does anyone have any examples ?

Regards,

-- Damien Bihel R&D (Research & Development) Engineer at ELRA (European Language Resources Association) E-Mail: biheldamien@gmail.com

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