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 <dominique.devriese@cs.kuleuven.be> wrote:
Gregory,
2010/10/3 Gregory Crosswhite <gcross@phys.washington.edu>:
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 ;)
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