[Haskell-cafe] List Monads and non-determinism

[Matt Ford]

Hi,

Thanks for the help.

I thought >>= was left associative?  It seems to be in the examples from Learn You A Haskell.

I tried to use the associative law to bracket from the right but it didn't like that either...

[1,2] >>= (\x -> (\n -> [3,4])) x  >>= \m -> return (n,m))

Any thoughts?

Matt 

On 19 Jul 2013, at 23:35, Rogan Creswick <creswick at gmail.com> wrote:

> On Fri, Jul 19, 2013 at 3:23 PM, Matt Ford <matt at dancingfrog.co.uk> wrote:
>> 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.
> 
> You're bracketing from the wrong end, which your intuition about n's visibility hints at.  Try this as your first set of parens:
> 
>  [1,2] >>= (\n -> [3,4] >>= \m -> return (n,m))
> 
> --Rogan
>  
>> 
>> 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.
>> 
>> Cheers,
>> 
>> --
>> Matt
>> 
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