Okay, so:
(defn evaltree [tree]
(cons (eval tree) (rest tree)))
That evaluates the tree, assuming that (rest tree) only contains numbers.
(eval-tree '(+ 3 9))
However, we also want to evaluate the arguments in the same way. The
arguments are a list, and when we want to do something to every item of a
list, we use a map or for loop:
(defn evaltree [tree]
(cons (eval tree) (map (fn [arg] ...) (rest tree))))
This is going to get a little long for one line, so let's spread it out on
multiple lines
(defn evaltree [tree]
(cons
(eval tree)
(map
(fn [arg] ...)
(rest tree))))
The question is, what should be in that mapped function? Well, if the
argument is not a list, then we don't need to do anything:
(defn evaltree [tree]
(cons
(eval tree)
(map
(fn [arg]
(if (list? arg)
...
arg))
(rest tree))))
And if the argument is a list... well, we already have a function do deal
with that: evaltree!
(defn evaltree [tree]
(cons
(eval tree)
(map
(fn [arg]
(if (list? arg)
(evaltree arg)
arg))
(rest tree))))
The disadvantage of this particular implementation is that, while simple,
it evaluates the inner forms more than once. This means it's probably not
as efficient as it could be, and side-effectful functions would trigger
multiple times.
- James
On 14 November 2015 at 19:40, <stevegarrid1...@gmail.com> wrote:
> Ok by starting with the basic (Def a '(+ 3 9))
>
>
> I have
> (Defn evaltree [tree]
> (Cons (eval tree) tree))
>
> But I can't seem to get it to work or drop the plus from the list. I'm
> really struggling with this
>
> Thanks for your help
>
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