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You have to take what you can get

And sometimes you have to get it by any means necessary. If it’s a file, you could use Jonathan Stowes URI::FetchFile. Said module checks if any of four modules are available and takes the first that sticks to turn a URI into a file on disk. There is one interesting line in his code that triggered an ENODOC.

$type = try require ::($class-name);

Here require returns a type object of a class declared by a module with the same name then that module.

Checking roast for that neat trick and playing with the whole dynamic module magic made me realise, that we don’t really cover this in the docs. When I try do handle an ENODOC I like to start with an example that compiles. This time, we need two files.

# M.pm6
unit module M;
class C is export { method m { 'method C::m' } };
class D is export { method m { 'method D::m' } };
# dynamic-modules.p6
use v6;
use lib '.';

subset C where ::('M::C');

my C $context = try { 
    CATCH { default { .note } };
    require ::('M');
    ::('M::C')
};

dd $context.HOW.^methods.elems;
dd $context.HOW.shortname($context);

Any symbol that is loaded via require will not be available at runtime. Consequently, we can’t have static type checks. Using a subset and dynamic lookup, we can get us a type object to check against. The where-clause will smart match against the type object. Since dynamic lookups are slow it may be sensible to cache the type object like so:

subset C where $ //= ::('M::C');

Now we got a type constraint to guard against require not returning a type that matches the name we expect. Please note we check against a name, not a type or interface. If you have the chance to design the modules that are loaded dynamically, you may want to define a role (that may even be empty) that must be implemented by the classes you load dynamically, to make sure you can actually call the methods you expect. Not just methods with the same name.

Now we can actually load the module by its name and resolve one of the classes dynamically and return it from the try block. Since M.pm6 defined a Module (as in Perl6::Metamodel::ModuleHOW) as its top level package, we can’t just simply take the return value of require because Module is not the most introspective thing we have in Perl 6. Please note that the symbols loaded by require are available via dynamic lookup outside the try-block. What happens if you go wild and load modules that have symbols with the same fully qualified name, I do not know. There may be dragons.

The case of loading any of a set of modules that may or may not be installed is quite a general one and to my limited knowledge we don’t got a module for that in the ecosystem yet. I therefore would like to challenge my three reads to write a module that sports the following interface.

sub load-any-module(*%module-name-to-adapter);
load-any-module({'Module::Name' => &Callable-adapter});

Whereby Callable-adapter provides a common interface to translate one sub or method call of the module to whatever user code requires. With such a module Jonathan may be able to boil URI::FetchFile down to 50 lines of code.

UPDATE:

Benchmarking a little revealed that `$` is not equivalent to `state $` while it should. To get the speedup right now use the following.

subset C where state $ = ::('M::C');

UPDATE 2:

The behaviour is by design and has been documented.

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Categories: Perl6
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  1. October 31, 2016 at 21:11

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