Add mapSearchT

[ocharles]

No description provided.

[ocharles]

I think you could also depend on mmorph here if you wanted, but I went with a transformers-like solution. I want this change because I want to have part of my search operate in the monad SearchT c (Accum x) - the idea is I can map over a list, but look at my accumulated state and make sure I'm not trying something I've already tried in a different branch. This is slightly different to AccumT x (Search c), which would fork the accumulator at each branch.

Also, thank you for this terrific library. We're putting it to good use at circuithub.com! If you would like help maintaining this library, just let me know and I'll be more than happy to help.

[ocharles]

Alas, I don't think this actually does what I want:

> runSearch $ evalAccumT ((lift (cost' (Sum 1)) >> add ['a'] >> look) <|> (lift (cost' (Sum 2)) >> add ['b'] >> look)) []
[(Sum {getSum = 1},"a"),(Sum {getSum = 2},"b")]

> runSearch $ mapSearchT (flip evalAccumT []) $ (cost' (Sum 1) >> lift (add ['a']) >> lift look) <|> (cost' (Sum 2) >> lift (add ['b']) >> lift look)
(Sum {getSum = 1},"a"),(Sum {getSum = 2},"b")]

I was hoping that the latter would have "ab" accumulated for both branches, but it dosen't seem to work how I think.

This does work:

> runAccum (runSearchT ((cost' (Sum 1) >> lift (add ['a']) >> lift look) <|> (cost' (Sum 2) >> lift (add ['b']) >> lift look))) []
([(Sum {getSum = 1},"a"),(Sum {getSum = 2},"ab")],"ab")

But I don't want to leave the Search monad. I imagine what I'll want is to instead produce a [Search c a] to see all branches taken instead.

Back to the drawing board!