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

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

Achim Schneider wrote:

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Achim Schneider wrote:

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[...] 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. jcc