20 Jul 00:23 2013

## List Monads and non-determinism

Matt Ford <matt <at> dancingfrog.co.uk>

2013-07-19 22:23:03 GMT

2013-07-19 22:23:03 GMT

Hi All, I thought I'd have a go at destructing [1,2] >>= \n -> [3,4] >>= \m -> return (n,m) which results in [(1,3)(1,4),(2,3),(2,4)] I started by putting brackets in ([1,2] >>= \n -> [3,4]) >>= \m -> return (n,m) This immediately fails when evaluated: I expect it's something to do with the n value now not being seen by the final return. It seems to me that the return function is doing something more than it's definition (return x = [x]). If ignore the error introduced by the brackets I have and continue to simplify I get. [3,4,3,4] >>= \m -> return (n,m) Now this obviously won't work as there is no 'n' value. So what's happening here? Return seems to be doing way more work than lifting the result to a list, how does Haskell know to do this? Why's it not in the function definition? Are lists somehow a special case? Any pointers appreciated.(Continue reading)