Hi Alex,
thanks for a thorough explanation as always!
cheers
On Tue, Aug 16, 2011 at 5:01 AM, Alexander Burger <a...@software-lab.de> wrote:
> Hi Edwin,
>
>> need some help. when is evList2() called in minipicolisp?
>> i can't decipher when it is called.
>> comments are mine.
>>
>> if (isNum(foo = car(ex)))
>> return ex; /* return numbers as is */
>
> Right. This is the case where the evaluated list starts with a number,
> e.g. (1 2 3 4), or also (123 a b c).
>
>
>> if (isCell(foo)) {
>
> 'foo' is the CAR of the list, and it is a list here. We might have a
> call like ((get 'x 'y) 1 2 3), so 'foo' is (get 'x 'y) which - when
> evaluated - is supposed to return a function.
>
>
>> if (isNum(foo = evList(foo)))
>> return evSubr(foo, ex); /* if a list evaluates to a number,
>> call a C subroutine */
>
> Yes. Evaluation of (get 'x 'y) resulted in a number, so a C function is
> called.
>
>> return evList2(foo, ex); /* then... ? */
>
> 'foo' is not a number, so these are the remaining possibilities: It must
> be either a symbol or a list. It might be a symbol if we have e.g. (x 1
> 2 3) where the value of 'x' is 'y', which in turn is defined as a
> function (or something else suitable to a recursive invocation of the
> whole 'evList' machinery). Otherwise, it is a list which is taken as a
> function (i.e. (<params> . <body>).
>
>
> 'evList2' is put into a separate function for a single reason: The value
> must be pushed on the stack to be safe from garbage collection (the
> 'Push' marcro at the start of 'evList2').
>
> This problem we have only in the C version, because the compiler
> allocates space in the stack frame for 'c1' on every call to 'evList',
> which is a waste of stack space in most cases, as "normal" function
> calls don't pass through here.
>
> Cheers,
> - Alex
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