[Haskell-beginners] Re: List Function

Heinrich Apfelmus apfelmus at quantentunnel.de
Thu Apr 30 03:57:25 EDT 2009

Edward Z. Yang wrote:
>> However, explaining how/why sequence does this requires understanding
>> the list monad, which you may or may not want to tackle at this point.
> Interesting! According to Hoogle, the sequence function evaluates each
> monad and then collects the results. I know evaluating a list monad returns
> a list, but what do they mean by "collect"?

There is also  sequence_  which does not collect the results, but throws
them away.

    Prelude> sequence_ [ [1,2,3], [4,5,6] ]

(Also, concerning terminology,  sequence  evaluates each monadic action.
You can't evaluate a monad, it's a type constructor, not a function or

>> If you import 'Control.Applicative' you can even do exactly what you
>> wanted, with pairs and all:
>>   Prelude Control.Applicative> liftA2 (,) [1,2,3] [4,5,6]
>>   [(1,4),(1,5),(1,6),(2,4),(2,5),(2,6),(3,4),(3,5),(3,6)]
>> Here, liftA2 is applying the function (,) to every possible pair of
>> values from the two lists.
> I'm not even going to try to understand that now.

Also known as

    liftM2 (,) [1,2,3] [4,5,6]

Which is the same as

       x <- [1,2,3]
       y <- [4,5,6]
       return (x,y)



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