Re: [Haskell-cafe] Suggestions for improvement

I suggest to pay more attention to haskell's standard library. "allButLast" is called "init" in Data.List module. Second, do not use explicit recursion. You can capture recursion using some high-order function like map, filter, foldr and so on: lastToTheLength xs = map f xs where f = const . last $ xs And last, your type signatures are too restrictive. You can apply your functions to arbitrary lists. lastToTheLength :: [a] -> [a] Standard library knowledge is very helpful in producing short and clear definitions. blowup = concat . zipWith replicate [1..] On Mon, Oct 4, 2010 at 1:24 AM, Dominique Devriese wrote:

Gregory,

2010/10/3 Gregory Crosswhite :

...

 On 10/3/10 1:45 PM, Dominique Devriese wrote:

...

Additionally, you can't combine the functions (blowup . allButLast) and lastToTheLength into a function that returns a pair like you seem to attempt. You need a function like the following for that:

comma :: (a ->  b) ->  (a ->  c) ->  a ->  (b,c) comma f g x = (f x, g x)

Then you could say:

blowup = (uncurry (++)) . comma (blowup . allButLast) lastToTheLength

It is worth noting that such a function already exists in the standard libraries;  it is the &&& operator in Control.Arrow:

   blowup = uncurry (++) . (blowup . allButLast &&& lastToTheLength)

Or you can write it as (liftA2 (,)) as I noted a few lines further in my mail ;)

Dominique _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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