[plt-scheme] HTDP 21.1.2 - trying to put it together

wooks . @ 2006-7-15 7:52:12 PM
"[plt-scheme] HTDP 21.1.2 - trying to put it together" <mid:BAY103-F568FFDE8392B4A82B38C1C06C0 at phx.gbl>

<SNIP>

> ;fold (A [listof Y] -> [listof Y])  [listof Y] [listof A] -> [listof Y]
> 
> thus far defining map in terms of fold gives us

> (define (map f a-list)
>    (fold someFunc empty a-list))

> Now to define someFunc.

> Well its contract must conform to

> ;someFunc: X [listof Y] -> [listof Y]

All of what you've said so far looks good to me.

> If we plug in Y for X and consider that Y is supposed to result from 
> applying  (the function we are mapping) to X
> then someFunc begins to look like this

> (define (someFunc f aloy)
>    (..... f ..... (first aloy)  (rest aloy))

> where f is a function with a contract

> (X -> Y)

> To produce a list as required by the contract  we have to use cons 
> somewhere.

Yes.

> I would have said

> (define (someFunc f aloy)
>     (cons (f first aloy) (rest aloy)))

> but it's not right - is it.

What's the contract of someFunc now? Does it match (A [listof Y] ->
[listof Y])?

If you look at someFunc's contract, it is very close, but needs to be
"flattened" (I'm not sure if that's technically the correct word.). Is
there some way to flatten it?

-- 
Chris Warrington <chrisw at rice.edu>

If a vegetarian eats vegetables, what does a humanitarian eat?
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