On Thursday, 26 July 2018 at 13:27:09 UTC, Alex wrote:
On Thursday, 26 July 2018 at 11:54:39 UTC, Mr.Bingo wrote:
The string itself could be useful however... Whatever OP has
in mind with this string...
Having a code block is useful in many ways simply because not
having it is the most limiting case. If one doesn't have it
and requires it then it is impossible for them to do what they
want. If one does have it and has no need, then no loss. Lots
of people like to err on the side of "caution" which is really
the side of limitations and frustrations and laziness. Of
course, sometimes things should be limited, but I think this
is probably not one of those cases(I see no harm in allowing
meta code to get a function body and, say, create another
function based off it but a slight change.
For example, one could mutate algorithms and run genetics
algorithms on them to see how function scan evolve. This might
lead to genetic ways to assemble programs. In any case, one
can't do it since one can't get at a functions body.
For example, maybe one wants a way to debug a mixin:
mixin("int x;"); <- can't debug
void foo()
{
int x;
}
mixin(funcBody!foo) <- can debug since we defined foo outside
of the mixin and the compiler see's it naturally.
I'm with you in this part.
I would argue, just because of "type safety" you mentioned, D
should not allow one to get the function body. The type
safety is not achievable because of
https://en.wikipedia.org/wiki/Lambda_calculus#Undecidability_of_equivalence
What the hell does that have to do with anything? I don't know
what you are talking about when it comes to "type safety" and
equivalence. What I am talking about is being able to parse
the function body in a type safe way rather than having to
hack strings or write a D parser oneself.
Instead of
"int x;"
one has <expression, <int, "x">>
or whatever the parser parses in to(some type of AST).
I'll try to reformulate this.
Take the example given at
https://dlang.org/spec/function.html#closures
Paragraph 2.
Now, examine the contents of functions abc and def.
Does the functionality differ? For sure not.
Does the compiler knows it? It is not guaranteed.
The article I cited and the literature behind it state, that in
general, an equivalence of function contents can not be
tracked. However, to state for a function to be equal with
another function exactly this is needed. And I thought you
meant this by "type safety"...
But this doesn't matter... If the idea is to return a string then
it doesn't matter, just return the requested function body. If is
to return an AST then it just gives(in whatever suitable form)
the AST for the requested function...
It's really not complicated:
Suppose one could get the function body as a string and suppose
one had a D parser. Just use the D parser on the string and
return the AST. D already has the D parser in it and also already
has the string in it. It's just a matter of someone exposing that
to the programmer to use. There is no issue anywhere because the
function cannot change while compiling(hence there can be no sync
issues).
I mean, if we want to see the function body we can just open up a
text editor... If we want to know the syntactical structure we
just parse the code with our brain. All that is being asked is to
have the compiler give us the string(easy) and parse it for us in
to some easy to use structure(some work but not hard, in fact, it
might be easy).
There are no function pointers to deal with, no compiling, etc.
Again, we can already do this stuff by hand by using import to
import the file containing the function we want to get... and use
a D parser to find the function proper and then grab the function
body and return it as a string. It is not hard to do but does
require using -J on the source path.
Here is a simple dumb way to get the function body. The problem
with "library" solutions is that they are not robust.
import std.stdio;
int foo(int x)
{
return x;
}
auto GetFunctionBody(alias B)()
{
import std.traits, std.meta, std.typecons, std.algorithm,
std.string, std.range;
enum ft = typeof(B).stringof;
enum rt = ReturnType!(B).stringof;
enum fn = fullyQualifiedName!(B);
enum n = split(fn, ".")[$-1];
enum mn = join(split(fn, ".")[0..$-1])~".d";
enum fs = rt~" "~n~ft[rt.length..$];
enum file = import(mn);
pragma(msg, n, ", ", ft, ", ", rt, ", ", fn, ", ", mn, ", ", fs);
auto res = "";
auto found = false;
for(int i = 0; i < file.length - fs.length; i++)
{
if (file[i..i+fs.length] == fs)
found = true;
if (found)
res ~= file[i];
if (file[i] == '}')
found = false;
}
return res;
}
int main()
{
writeln(GetFunctionBody!(foo));
getchar();
return 0;
}
