USE, but fail nicely

vaevictus has asked for the wisdom of the Perl Monks concerning the following question:

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Re: USE, but fail nicely by tadman (Prior) on Feb 01, 2001 at 04:22 UTC
`#!/usr/bin/perl -w use strict; eval qq{ use Time::HiRes qw(time); }; print "\n";` [download] Two characters, my friend. eval with braces forces compilation before "evaluation", which makes it function more like do{}. If you pass a scalar, it is compiled after the main program and any compilation errors are directed to the program, not your STDERR. This is the difference between 'eval EXPR' and 'eval BLOCK'. The Camel has a lot to say about the Joy of Eval, and so does 'perldoc -f eval'. Note that this will "do absolutely nothing" as you requested. Errors from compilation show up in $@ and you can warn, or act accordingly.	[reply] [d/l]
Re: USE, but fail nicely by btrott (Parson) on Feb 01, 2001 at 04:23 UTC
You do want eval, but not quite like that; unless you put the use into a string, it'll still get run at compile-time and will die anyway. You want: `eval "use Time::HiRes qw(time)"; die "Nice message: $@" if $@;` [download]	[reply] [d/l]
Re: USE, but fail nicely by extremely (Priest) on Feb 01, 2001 at 04:33 UTC
try: `eval 'use Time::HiRes qw(time)';` The `eval {};` form actually tokenizes what is inside the brackets and thus the parser will catch `use, no, BEGIN {}, and END{}` pragmas and blocks. The string form is just a string till the eval actually happens at runtime. Try these at the commandline to see the various behaviors: `perl -e 'if(shift){eval{BEGIN{print"eek\n"}print"ahh\n"}}' 0 perl -e 'if(shift){eval{BEGIN{print"eek\n"}print"ahh\n"}}' 1 perl -e 'if(shift){eval"BEGIN{print\"eek\n\"}print\"ahh\n\""}' 0 perl -e 'if(shift){eval"BEGIN{print\"eek\n\"}print\"ahh\n\""}' 1` [download] -- $you = new YOU; honk() if $you->love(perl)	[reply] [d/l] [select]
Re: USE, but fail nicely by Fastolfe (Vicar) on Feb 01, 2001 at 05:38 UTC
I might attempt this like this, which is a little more verbose than the string version of eval, but "feels" more correct to me (though there's nothing wrong with using that other form of eval): `BEGIN { eval { require Time::HiRes; import Time::HiRes 'time'; } warn "Couldn't load Time::HiRes, but that's OK\n\t$@" if $@; }` [download]	[reply] [d/l]
(crazyinsomniac) Re: USE, but fail nicely by crazyinsomniac (Prior) on Feb 01, 2001 at 04:22 UTC
update: After realizing what i did wrong, i changed it to what it was meant to do. `#!perl -w use strict; eval("use Time::HiRes qw(time);"); if($!){die "blah:$!";}` [download] "cRaZy is co01, but sometimes cRaZy is cRaZy". - crazyinsomniac	[reply] [d/l]
Re: USE, but fail nicely by Trinary (Pilgrim) on Feb 01, 2001 at 04:23 UTC
use (IIRC) pretends to be a BEGIN block...use `eval "require Mod::Ule; import Mod::Ule";` Um...typos nonwithstanding. trinary	[reply] [d/l]