[Haskell-cafe] Re: The Worker/Wrapper Transformation

Jonathan Cast jonathanccast at fastmail.fm
Thu Jan 3 11:18:17 EST 2008

On 3 Jan 2008, at 4:49 AM, Isaac Dupree wrote:

> Achim Schneider wrote:
>> Achim Schneider <barsoap at web.de> wrote:
>>> [...]
>> I'm trying to grok that
>> [] = id
>> ++ = .
>> in the context of Hughes lists.
> they are also known as "difference lists", and also used at type  
> String in the Prelude as "ShowS", to help avoid quadratic behavior  
> when making complicated Strings.  the [a]->[a] is not an ordinary  
> function -- it's expected not to examine its argument, just to use  
> it exactly once (is there a formal way to say that?)

f xn = f [] ++ xn

is the first thing off the top of my head.

OTOH, examining your argument (while, strictly speaking unsafe) is  
pretty darn cool:

f [] = "foo"
f (c:s) | isAlphaNum c = "foo "++c:s
         | otherwise    = "foo"++c:s

Token prepend.


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