[Haskell-cafe] A beginners question

[Adam Langley]

On Sat, Feb 23, 2008 at 8:00 AM, Harri Kiiskinen <harkiisk at utu.fi> wrote:
>  then why does
>
>  let i = fmap (^4) [1,2,3] in shows i " "
>
>  give
>
>  "[1,16,81] "

I'll probably mess this up somewhere, but if I do, reset assured that
someone else here will correct me ;)

fmap (^4) [1,2,3] == [1,16,81] so shows " " of that == "[1,16,81] "
(note the trailing space), obvious I hope.

However, [1,16,81] >>= \i -> shows i " " operates in the list monad. I
can't find the actual instance of Monad [] right now, but it will
apply 'shows i " "' to each element in the first list and concat the
results. Since strings are just [Char], concating them is string
concatenation. And so you have "1 16 81 " (again, note the trailing
space)


AGL

-- 
Adam Langley agl at imperialviolet.org http://www.imperialviolet.org