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I am always interested in learning new languages, a fact that keeps me on my toes and makes me (I believe) a better programmer. My attempts at conquering Haskell come and go - twice so far - and I decided it was time to try again. 3rd time's the charm, right? Nope. I re-read my old notes... and get disappointed :-( The problem that made me lose faith last time, was an easy one: permutations of integers. i.e. from a list of integers, to a list of lists - a list of their permutations: [int] -> [[int]] This is in fact a generic problem, so replacing 'int' above with 'a', would still apply. From my notes: I code it first on my own, I succeed. Hurrah! I send my solution to a good friend of mine - Haskell guru, it usually helps to learn from gurus - and he sends me this, which I am told, "expresses the true power of the language, the use of generic facilities to code your needs". All for it, I recently drank the kool-aid, let's go: permute :: [a] -> [[a]] permute = foldr (concatMap.ins) [[]] where ins x [] = [[x]] ins x (y:ys) = (x:y:ys):[ y:res | res <- ins x ys] Hmm. Let's break this down: bash$ cat b.hs ins x [] = [[x]] ins x (y:ys) = (x:y:ys):[ y:res | res <- ins x ys] bash$ ghci Prelude> :load b.hs [1 of 1] Compiling Main ( b.hs, interpreted ) Ok, modules loaded: Main. *Main> ins 1 [2,3] [[1,2,3],[2,1,3],[2,3,1]] OK, so far, so good. Took me a minute to understand the second line of "ins", but OK: It places the 1st arg in all possible positions in the list. Cool. Now, to understand the foldr and concatMap. in "Real world Haskell", the DOT was explained... (f . g) x ...as just another syntax for... f (g x) And in the code the guru sent, DOT was used from a foldr, with the "ins" function as the fold

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