In J (a sort of dialect of APL), there's a thing called "under", written "&.". The expression "(f &. g) x" is equivalent to "(g^:_1) (f (g x))" where "g^:_1" is J's "obverse" of g, which in cases where it exists is usually the inverse of g ( http://www.jsoftware.com/help/dictionary/intro26.htm). Abusing notation with some weird mixture of Haskell and J, this means that "((+) &. log)" multiplies numbers by taking logs, adding and exponentiating. You "inv" is "under" for cases where g == g^-1 (reverse being a good example). In cases where g /= g^-1, it's obviously a useful operation, but the case where g == g^-1 seems a bit specialised. Can you think of any other useful cases than g == reverse? I guess "inv (1/) sum" is the harmonic mean, but that's another special case.
On 17 August 2013 11:40, Mateusz Kowalczyk <fuuze...@fuuzetsu.co.uk> wrote: > On 17/08/13 10:11, Christopher Done wrote: > > Anyone ever needed this? Me and John Wiegley were discussing a decent > > name for it, John suggested inv as in involution. E.g. > First thing I thought was ‘inverse’… > > > > inv reverse (take 10) > > inv reverse (dropWhile isDigit) > > trim = inv reverse (dropWhile isSpace) . dropWhile isSpace > > > > That seems to be the only use-case I've ever come across. > > > I do this a lot as well. Why not skip the ‘g’ all together and have ‘f . > reverse . f’ if that's all we're doing? You could even call it fromEnd > at that point and we end up with a rather intuitive ‘fromEnd (drop 10)’. > Maybe even just have an operator. > > There's also this one: > > > > co f g = f g . g > > > > which means you can write > > > > trim = co (inv reverse) (dropWhile isSpace) > > > > but that's optimizing an ever rarer use-case. > > > > > Is this a proposal for addition to something or is it just general > discussion? > > > -- > Mateusz K. > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > > -- Ian Ross Tel: +43(0)6804451378 i...@skybluetrades.net www.skybluetrades.net
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