[Haskell-cafe] unapplying function definitions?
tom.davie at gmail.com
Sun May 4 11:52:50 EDT 2008
On 4 May 2008, at 17:33, PR Stanley wrote:
> What on earth is unapplying function definitions?
> The following is taken from chapter 13 of the Hutton book:
> "...when reasoning about programs, function definitions can be both
> applied from left to right and unapplied from right to left."
Well, because of referential transparency, we can say that the left
hand side of a function is exactly equal to the right hand side.
Thus, we can instead of applying functions, and making progress
towards a normal form, unapply them and get further away from a normal
form... for example:
5 = head [5,6,7,8,9] = head ([5,6] ++  ++ [8,9]) = head (( ++ 
++ ) ++  ++ [8,9]) .......
There are of course an infinite number of ways of doing this, so it's
usually only interesting, if we have some reason for applying a
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