Re: My scripting language - any suggestions?

On Aug 26, 1:41 am, lican <lica...@gmail.com> wrote:
> Anyway I'm having some
> difficulties. One small decision, should variables should be declared:
>
> [code]
> var a = 5; // or
> local a = 5; // like in lua or unreal script, or
> global a = 5; // declaration of a global variable in given scope, not
> like referencing global variables in PHP, or
> a = 5;
> [/code]


IMO, dynamic languages need to require variable declarations. I still
loathe the waste of time PHP caused because I missed a typo in a
variable name. That's something the compiler should catch. Or at least
handle unit test in a simple way like Ruby. I believe the reason typos
don't bite Ruby people so much because they discover them via unit
tests easily. Also ruby complains if you access a name which it hasn't
seen before. I can't remember what PHP does in that situation.


Regarding the way how to declare the variables: It depends on your
needs. If you plan to support proper class support you need to be able
to say which members are public, protected or private. There are two
ways: Either you put the modifier in front of each member (like Java/
C#) or you put the modifier in a separate line and say, that
everything afterwards has this modifier (like C++/Ruby). It depends to
a part on the need of proper declaration and to a part on the
aesthetics - in other words, if it looks ugly or not.


> And the second problem. What kind of scope to implement. In PHP you
> have either global or local (function) scope. So writing:
>
> [code]
> if( variable )
> {
> a = 5;
>
> }
>
> print(a);
> [/code]
>
> gives the result '5'. Something like this would mean a compiler error
> in C++. But I'm willing to implement it as per block (like in C/C++)
> scope. Anyone seeing any prons/cons?


Using scopes has the advantage to reduce the variable life time. If
you only need the variable a certain number of lines, then accessing
it later should give an error as it goes against declared intent. On
the other hand is separating declaration and assignment a tad ugly
(even if you don't use "var a", you have to put the symbol into the
symbol table somehow). Also you can shadow variables, if a declaration
before prior use is required. If you don't like that you can prevent
any total shadowing (unlike shadowing member variables which can still
be accessed via "this.a") like C# does (read its spec as it is a bit
more involved than I hinted on).


> The interpreter is a simple AST
> walking class, but when some problems are fixed I will replace it with
> a bytecode VM (like in Lua). And as for the VM itself... stack or
> register based? :)


IIRC, .NET uses stack because it makes it easier to verify bytecode.
But I haven't looked into VM design myself, so I can't say anymore on
this subject.


Johannes

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