Re: coderefs and (&) prototypes

Why does a (&) prototype only accept blocks or sub{} but not scalar coderefs?

It does. In fact, that's the only thing it accepts. The real question is why doesn't it accept arbitrary expressions that return a code ref. And the answer is that there's no way to guarantee that the expression will return a code ref. Rather than adding code to the optree to do run-time type checking, prototypes only allow operators that are guaranteed to return a value of the appropriate type.

For example,
The arg for (\@) must start with a @.
The arg for (\%) must start with a %.

sub {} is guaranteed to return a code ref, so it's allowed for (&).
\&cb is guaranteed to return a code ref, so it's allowed for (&). By extension, this includes \&{ EXPR }.

&foo, on the other hand, is a sub call. It can return anything. It's not allowed for (&) (or (\@) or (\%)).

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Re^2: coderefs and (&) prototypes by JadeNB (Chaplain) on Jul 28, 2009 at 21:20 UTC
The real question is why doesn't it accept arbitrary expressions that return a code ref. And the answer is that there's no way to guarantee that the expression will return a code ref. and then \&cb is guaranteed to return a code ref, so it's allowed for (&). By extension, this includes \&{ EXPR }. I don't understand these two together. There's no way to guarantee that `EXPR` in `\&{ EXPR }` will evaluate to a code-ref either, and, if it doesn't, the extra `\&` noise isn't going to prevent it from blowing up at run-time—so why put the programmer through that extra syntactic hassle, if it doesn't buy him or her any extra safety?	[reply] [d/l] [select]
Re^3: coderefs and (&) prototypes by ikegami (Patriarch) on Jul 28, 2009 at 21:23 UTC
the extra \& noise isn't going to prevent it from blowing up at run-time The whole point of parameter validation is to blow up. Why would you want to prevent it? Without the "extra noise", it wouldn't blow up. That's the problem. `sub f0 { my ($cb) = @_; '...' } sub f1(&) { my ($cb) = @_; '...' } f0(\&abc); # OK No error f0(\&{ maybe_coderef() }); # OK Error caught at run-time (if any) f0("abc"); # XX Error uncaught f0(maybe_coderef()); # XX Error uncaught (if any) f1(\&abc); # OK No error f1(\&{ maybe_coderef() }); # OK Error caught at run-time (if any) f1("abc"); # OK Error caught at compile-time f1(maybe_coderef()); # OK Error caught at compile-time (if any)` [download] Update: Added example	[reply] [d/l]