Match treats `empty` as an identifier to bind, not as a pattern for the empty
list. This means that the first case will always be matched. You can use
`(list)` or `'()` to match an empty list.
Also, in the last catch-all case, you pass the function `rest` which returns
the tail of a list to `filter-odds`. You probably wanted to shadow that binding
by using `(cons first rest)` instead of `_`.
Here's the version with the fixes above:
(define (filter-odds v)
(match v
[(list) (list)]
[(cons (? odd? first) rest)
(cons first (filter-odds rest))]
[(cons first rest) (filter-odds rest)]))
(filter-odds '(1 2 3 4 5)) ; ==> '(1 3 5)
Best,
Michael MacLeod
On Sunday, April 21, 2019 1:19:03 PM PDT Tim Meehan wrote:
> Forgive this naive question, I am having some trouble understanding some
> matching forms. If I wanted to filter out odd numbers like this:
>
> (filter odd? '(1 2 3 4 5)) => '(1 3 5)
>
> but I wanted to do this with a match:
> (define (filter-odds v)
> (match v
> [empty empty]
> [(cons (? odd? first) rest) (cons first (filter-odds rest))]
> [_ (filter-odds rest)]))
>
> (filter-odds '(1 2 3 4 5)) => '(1 2 3 4 5)
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