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From: BartC <bc@freeuk.com>
Newsgroups: comp.compilers
Subject: Re: I have managed to parse and evaluate using an AST. I got stuck in statements and block
Date: Sat, 21 Feb 2015 20:52:20 +0000
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On 19/02/2015 21:51, BartC wrote:
> On 18/02/2015 19:20, mehmet.coskun@gmail.com wrote:
>
>> Can you please help me to design if, while, statement and statement
>> block functions? How can I implement this? What do I miss here in my
>> implementation that caused me getting stuck without a working
>> interpreter.

> What you need for execution is some sort of program counter to tell you
> whereabouts in the AST you are, and what to execute next. I can't quite
> see that in the code you posted.

I had a go at taking an existing byte-code compiler and adapting the
byte-code generator to execute the code instead of producing the normal
output.

It was a lot harder than I expected! Certainly, to be able to randomly
jump from one part of the code to another is difficult. But to execute
your example, that can be done with almost the same AST-scanning method
I'd been using for the byte-code.

I started with this version of your example, but in the language this
compiler processes (which happens to be the same language the compiler
is written in):

proc start=
count:=0

while count<10 do
     println count
     if count=5 then
         println "High Five"
     fi
     count:=count+1
od
end

Then I only modified the small number of lines needed to get this to
execute. The While handler was:

proc eval_while (p,cond,body,c,d)=
while eval_expr(cond) do
     eval_block(body)
od
end

The If handler:

proc eval_if (p,cond,b,c,d) =
if eval_expr(cond) then
     eval_block(b)
elsif notnull(c) then  # else part is optional
     eval_block(c)
fi
end

So the while handler involves a while-loop, and the if-then-else handler
involves an if-then-else statement, which sort of makes sense.

For handling code-blocks and statements, I use:

proc eval_block (p)=
forall s in p.a do
     eval_stmt(s)
od
end

But this starts to depend on the details of how your AST is organised.
(I haven't shown assignment, since I haven't implemented variables
properly and it's just a kludge.)

The few operations I needed (<, =, +) are inside this function:

function eval_expr(p)=

switch p.tag
when j_const then
	return p.a

when j_name then        # variable name
	return p.a.code

when j_add then
	return eval_expr(p.a)+eval_expr(p.b)

when j_lt then
	return eval_expr(p.a)<eval_expr(p.b)

when j_eq then
	return eval_expr(p.a)=eval_expr(p.b)
......
end

But you seem to have that part sorted. The output from this is:

  0
  1
  2
  3
  4
  5
  High Five
  6
  7
  8
  9

So it seems to work. (To fully convert my project to execute ASTs is
quite a lot of work, and the simple approach above won't work. There is
also the problem of how to deal with programs consisting of multiple
modules - it would be necessary to store the AST in a file, a whole lot
of problems just to deal with that. For experimenting however it's fine.)

--
Bartc