[Haskell-cafe] Suggestions for improvement

Dominique Devriese dominique.devriese at cs.kuleuven.be
Sun Oct 3 16:45:30 EDT 2010

> One question I have is whether I can eliminate points in the above
> definition of blowup, and write something like
>    blowup = (++) . (blowup . allButLast, lastToTheLength)
> thinking of (++) as a function String x String -> String.

Actually (++) is of type String -> String -> String. When you want
something of the type you mean (you normally write that as (String,
String) -> String in Haskell, then you can use (uncurry (++)).

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

Ignore this if you haven't read about Applicative or type classes yet,
but using the Applicative instance for arrow types (->) a, you can
also write

comma = liftA2 (,)


blowup = (uncurry (++)) . liftA2 (,) (blowup . allButLast) lastToTheLength

> Also, I can't
> figure out whether it is possible to get a shorter solution using fold.
> I have tried Hlint on my file, but it gave no suggestions.
> I am sure there are better ways, and would like some pointers and any
> general suggestions for improvement.

By the way, shorter is not always better. Trying to recognize
abstraction patterns in your code is never a bad thing though.


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