<div dir="ltr"><div style><span style="font-family:arial,sans-serif;font-size:13px">(For some reason my previous post seems to have been truncated.)</span></div><span style="font-family:arial,sans-serif;font-size:13px"><div>
<span style="font-family:arial,sans-serif;font-size:13px"><br></span></div>I have a function that, simplifying a little, looks like this:</span><div style="font-family:arial,sans-serif;font-size:13px"><br></div><div style="font-family:arial,sans-serif;font-size:13px">
fn :: [a] -> a</div><div style="font-family:arial,sans-serif;font-size:13px"> fn = (head .) . takeWhile $ (\_ -> True)</div><div style="font-family:arial,sans-serif;font-size:13px"><br></div><div style="font-family:arial,sans-serif;font-size:13px">
>From this, I want a function with the signature fn' :: [(x,a)] -> (x,a) such that:</div><div style="font-family:arial,sans-serif;font-size:13px"><br></div><div style="font-family:arial,sans-serif;font-size:13px"> snd $ fn' (zip [x] [a]) = fn [a]</div>
<div style="font-family:arial,sans-serif;font-size:13px"><br></div><div style="font-family:arial,sans-serif;font-size:13px">I can see ways of doing this by altering fn, or by repeating some of fn in the definition of fn', or (because in this case I know that if fn xs = x, fn is returning the first x in xs and not any others), by doing something nasty like:</div>
<div style="font-family:arial,sans-serif;font-size:13px"><br></div><div style="font-family:arial,sans-serif;font-size:13px"> fn' xs = xs !! fromMaybe 0 (findIndex (\(_,a) -> a == fn (snd $ unzip xs)) xs )<br></div>
<div style="font-family:arial,sans-serif;font-size:13px"><br></div><div style="font-family:arial,sans-serif;font-size:13px">But it seems to me like there should be prettier solutions to this (that <i>do not</i> involve changing the definition of fn). After all, this doesn't seem like a rare pattern.</div>
<div style="font-family:arial,sans-serif;font-size:13px"><br></div><div style="font-family:arial,sans-serif;font-size:13px">Anyone know if in fact there's a better way to go about it?</div></div>