[Haskell-cafe] Reducing code for efficient ShowS

[wren ng thornton]

Sean Leather wrote:
> There's the obvious approach that's described in every tutorial, book, and
> research paper (for didactic purposes, of course).
>
>> ex1 = "(" ++ show n ++ " " ++ s ++ ")"
>
> It's pretty concise, but it's horribly inefficient due to the use of (++).
>
> Then, there's the ShowS approach.
>
>> ex2 = showChar '(' . shows n . showChar ' ' . showString s . showChar ')'
> $ ""
>
> This is more efficient, but now the code has bloated up a lot.
>
> Why can't I have my cake and eat it, too? I want to write with as little
> code as
> |ex1| (or less if possible), and I want it to be as efficient as |ex2|.
>
> I propose this example as an improvement.
>
>> ex3 = '(' .+. n .+. ' ' .+. s .$. ')'

Why not use the dlist library:

    http://hackage.haskell.org/cgi-bin/hackage-scripts/package/dlist

With something like (untested code):

 > xs +++ ys = shows    xs `append` shows    ys
 > x  .++ ys = showChar x  `cons`   shows    ys
 > xs ++. y  = shows    xs `snoc`   showChar y
 >
 > ext3' = toList $ '(' .++ n +++ ' ' .++ s ++. ')'


-- 
Live well,
~wren