digitalmars.D.learn - How to check that import module will succeed?
- Andrey Zherikov (15/15) Jul 25 2019 Is there a way to check whether some module, say "foo", is
- evilrat (18/20) Jul 25 2019 I did some really retarded utility like this in the past, worked
- Anonymouse (6/17) Jul 26 2019 I forgot the exact details but I ran into troubles with this
- Andrey Zherikov (5/7) Jul 26 2019 I couldn't find "identifier" docs on
- Andrey Zherikov (35/55) Jul 26 2019 This works, thanks!
- evilrat (23/37) Jul 26 2019 Exactly. Because of scoping rules mixin templates are nearly
- Paul Backus (8/23) Jul 26 2019 version(HasStdio)
- Andrey Zherikov (2/9) Jul 26 2019 I want to achieve similar result without versions.
- Adam D. Ruppe (4/6) Jul 26 2019 static if (is(typeof((){import that.module.here;}))) {
Is there a way to check whether some module, say "foo", is
available for import before doing "import foo"?
I want to create a function that imports module if it's available
or does something else otherwise. So I think the code should look
something like this:
mixin template my_import(alias module_name)
{
if(module_name is available)
mixin("import "~module_name~";");
else
pragma(msg, module_name~" is not available");
}
mixin my_import!("std.stdio"); // == import std.stdio
mixin my_import!("unknown_module"); // == pragma(msg,
"unknown_module is not available")
Jul 25 2019
On Friday, 26 July 2019 at 03:42:58 UTC, Andrey Zherikov wrote:Is there a way to check whether some module, say "foo", is available for import before doing "import foo"?I did some really retarded utility like this in the past, worked for me, but I can't say it is that well tested and there might be a better way (after all it just assumes that template instantiation failure can only mean there is no such module), so in short template allowed to fail, and we check if it is failed or not to test if desired module exists. Don't remember if static is really necessary or you can just return directly. bool isModuleAvailable(alias modName)() { mixin("import " ~ modName ~ ";"); static if (__traits(compiles, mixin(modName).stringof)) return true; else return false; } // use like this static if (__traits(compiles, isModuleAvailable!"mymod" )) import mymod;
Jul 25 2019
On Friday, 26 July 2019 at 06:24:18 UTC, evilrat wrote:
On Friday, 26 July 2019 at 03:42:58 UTC, Andrey Zherikov wrote:
bool isModuleAvailable(alias modName)() {
mixin("import " ~ modName ~ ";");
static if (__traits(compiles, mixin(modName).stringof))
return true;
else
return false;
}
// use like this
static if (__traits(compiles, isModuleAvailable!"mymod" ))
import mymod;
I forgot the exact details but I ran into troubles with this
where __traits(compiles, moduleName) would evaluate to false even
if the module was available, if there were top-level errors in it.
I use __traits(compiles, __traits(identifier, moduleName)) now
instead and it seems to work.
Jul 26 2019
Andrey Zherikov <andrey.zherikov gmail.com> On Friday, 26 July 2019 at 14:14:11 UTC, Anonymouse wrote:I use __traits(compiles, __traits(identifier, moduleName)) now instead and it seems to work.I couldn't find "identifier" docs on https://dlang.org/phobos/std_traits.html. But I tried and it doesn't work - I think because <moduleName> is unknown identifier until "import <moduleName>" is happened.
Jul 26 2019
On Friday, 26 July 2019 at 06:24:18 UTC, evilrat wrote:On Friday, 26 July 2019 at 03:42:58 UTC, Andrey Zherikov wrote:This works, thanks! But when I try to wrap usage as a single call to something I get slightly different result of import: // mymod.d module mymod; void myfunc() { import std.stdio; writeln("myfunc"); } // use1.d static if (__traits(compiles, isModuleAvailable!"mymod" )) import mymod; pragma(msg,fullyQualifiedName!(myfunc)); // mymod.myfunc pragma(msg,fullyQualifiedName!(mymod.myfunc)); // mymod.myfunc // use2.d mixin template my_import(alias modName) { static if (__traits(compiles, isModuleAvailable!modName )) mixin("import " ~ modName ~ ";"); } mixin my_import!"mymod"; pragma(msg,fullyQualifiedName!(myfunc)); // Error: undefined identifier myfunc pragma(msg,fullyQualifiedName!(mymod.myfunc)); // mymod.myfunc Even without static if I get the same result: mixin template my_import(alias modName) { mixin("import " ~ modName ~ ";"); } mixin my_import!"mymod"; pragma(msg,fullyQualifiedName!(myfunc)); // Error: undefined identifier myfunc pragma(msg,fullyQualifiedName!(mymod.myfunc)); // mymod.myfunc If I understood template mixin doc correctly this happens because of "The declarations in a mixin are placed in a nested scope". So is there a way to make two use cases above to work the same way, i.e. "myfunc" to be available without "mymod." prefix?Is there a way to check whether some module, say "foo", is available for import before doing "import foo"?I did some really retarded utility like this in the past, worked for me, but I can't say it is that well tested and there might be a better way (after all it just assumes that template instantiation failure can only mean there is no such module), so in short template allowed to fail, and we check if it is failed or not to test if desired module exists. Don't remember if static is really necessary or you can just return directly. bool isModuleAvailable(alias modName)() { mixin("import " ~ modName ~ ";"); static if (__traits(compiles, mixin(modName).stringof)) return true; else return false; } // use like this static if (__traits(compiles, isModuleAvailable!"mymod" )) import mymod;
Jul 26 2019
On Friday, 26 July 2019 at 14:56:37 UTC, Andrey Zherikov wrote:
Even without static if I get the same result:
mixin template my_import(alias modName)
{
mixin("import " ~ modName ~ ";");
}
mixin my_import!"mymod";
pragma(msg,fullyQualifiedName!(myfunc)); // Error:
undefined identifier myfunc
pragma(msg,fullyQualifiedName!(mymod.myfunc)); // mymod.myfunc
If I understood template mixin doc correctly this happens
because of "The declarations in a mixin are placed in a nested
scope". So is there a way to make two use cases above to work
the same way, i.e. "myfunc" to be available without "mymod."
prefix?
Exactly. Because of scoping rules mixin templates are nearly
useless, there was a lot of frustration/criticism on them, but
this is by design - to not turn them in macro hell like in C.
My opinion is that if core team are so picky about explicitness
they should also provide a way to explicitly mix into the current
scope, but of course I'm just yet another stranger here.
So you can do string mixin instead, however it requires more
labor, and usually one has to keep in mind all intricacies of the
features used which quickly leads to complexity combinatory
explosion.
But whatever... With helper function above you can try this
property string my_import(alias mod)()
{
return
"static if (__traits(compiles, isModuleAvailable!\"" ~ mod ~
"\"))" ~
"{" ~
" mixin(\"import \",\"" ~ mod ~ "\", \";\"); " ~
"}";
}
// should import into current scope
mixin(my_import!"std.string");
Jul 26 2019
On Friday, 26 July 2019 at 03:42:58 UTC, Andrey Zherikov wrote:
Is there a way to check whether some module, say "foo", is
available for import before doing "import foo"?
I want to create a function that imports module if it's
available or does something else otherwise. So I think the code
should look something like this:
mixin template my_import(alias module_name)
{
if(module_name is available)
mixin("import "~module_name~";");
else
pragma(msg, module_name~" is not available");
}
mixin my_import!("std.stdio"); // == import std.stdio
mixin my_import!("unknown_module"); // == pragma(msg,
"unknown_module is not available")
version(HasStdio)
import std.stdio;
else
pragma(msg, "std.stdio is not available");
Then configure your build system to pass `-version=HasStdio` to
dmd (or the equivalent flag to ldc or gdc) when std.stdio is
available.
Jul 26 2019
On Friday, 26 July 2019 at 14:19:05 UTC, Paul Backus wrote:
version(HasStdio)
import std.stdio;
else
pragma(msg, "std.stdio is not available");
Then configure your build system to pass `-version=HasStdio` to
dmd (or the equivalent flag to ldc or gdc) when std.stdio is
available.
I want to achieve similar result without versions.
Jul 26 2019
On Friday, 26 July 2019 at 03:42:58 UTC, Andrey Zherikov wrote:Is there a way to check whether some module, say "foo", is available for import before doing "import foo"?static if (is(typeof((){import that.module.here;}))) { // it is available }
Jul 26 2019