What really fails in installation?

I've finally installed Perl 5.10.1 and I had to reinstall most of the modules I use. I set PERL_MM_USE_DEFAULT=1 and did make for a module that requires half of the modules on my computer. Everything was installed automatically - with no additional intervention from me. The morale of this is that if you are using Ubuntu and if you have installed all the external prerequisites - then the installation is really rather automatic. The problem with systems other than a special flavour of Linux is both hard to solve and rather unsophisticated - we just need more people working on those systems and reporting and solving bugs. Or at least that is the solution I can imagine. But the problem with external prerequisites is where we could do better with just a bit of programming.

I remove memcached from my system and try

zby@zby:~/progs/Enzyme$ cpanp i Cache::Memcached::Managed
Installing Cache::Memcached::Managed (0.20)
Running [/home/zby/local/bin/perl /usr/local/bin/cpanp-run-perl /home/zby/.cpanplus/5.10.1/build/Cache-Memcached-Managed-0.20/Makefile.PL]...
No executable memcached found: No such file or directory
Running [/usr/bin/make test]...
PERL_DL_NONLAZY=1 /home/zby/local/bin/perl "-MExtUtils::Command::MM" "-e" "test_harness(0, 'blib/lib', 'blib/arch')" t/*.t
Can't exec "memcached": No such file or directory at /home/zby/.cpanplus/5.10.1/build/Cache-Memcached-Managed-0.20/blib/lib/Cache/Memcached/Managed.pm line 843.
#
# Stopped memcached server
# Looks like you planned 176 tests but ran 184.
t/001basic.t .....
Dubious, test returned 255 (wstat 65280, 0xff00)
All 176 subtests passed
(less 177 skipped subtests: -1 okay)
t/002inactive.t .. ok
t/003multi.t ..... ok
Can't exec "memcached": No such file or directory at /home/zby/.cpanplus/5.10.1/build/Cache-Memcached-Managed-0.20/blib/lib/Cache/Memcached/Managed.pm line 843.
#
# Stopped memcached server
t/010fork.t ...... ok
Can't exec "memcached": No such file or directory at /home/zby/.cpanplus/5.10.1/build/Cache-Memcached-Managed-0.20/blib/lib/Cache/Memcached/Managed.pm line 843.
#
# Stopped memcached server(s)
t/020grab.t ...... ok

Test Summary Report
-------------------
t/001basic.t (Wstat: 65280 Tests: 184 Failed: 8)
Failed tests: 177-184
Non-zero exit status: 255
Parse errors: Bad plan. You planned 176 tests but ran 184.
Files=5, Tests=36330, 18 wallclock secs ( 6.12 usr 0.03 sys + 3.86 cusr 0.20 csys = 10.21 CPU)
Result: FAIL
Failed 1/5 test programs. 8/36330 subtests failed.
make: *** [test_dynamic] Error 255
[ERROR] MAKE TEST failed: Bad file descriptor PERL_DL_NONLAZY=1 /home/zby/local/bin/perl "-MExtUtils::Command::MM" "-e" "test_harness(0, 'blib/lib', 'blib/arch')" t/*.t
Can't exec "memcached": No such file or directory at /home/zby/.cpanplus/5.10.1/build/Cache-Memcached-Managed-0.20/blib/lib/Cache/Memcached/Managed.pm line 843.
#
# Stopped memcached server
# Looks like you planned 176 tests but ran 184.
t/001basic.t .....
Dubious, test returned 255 (wstat 65280, 0xff00)
All 176 subtests passed
(less 177 skipped subtests: -1 okay)
t/002inactive.t .. ok
t/003multi.t ..... ok
Can't exec "memcached": No such file or directory at /home/zby/.cpanplus/5.10.1/build/Cache-Memcached-Managed-0.20/blib/lib/Cache/Memcached/Managed.pm line 843.
#
# Stopped memcached server
t/010fork.t ...... ok
Can't exec "memcached": No such file or directory at /home/zby/.cpanplus/5.10.1/build/Cache-Memcached-Managed-0.20/blib/lib/Cache/Memcached/Managed.pm line 843.
#
# Stopped memcached server(s)
t/020grab.t ...... ok

Test Summary Report
-------------------
t/001basic.t (Wstat: 65280 Tests: 184 Failed: 8)
Failed tests: 177-184
Non-zero exit status: 255
Parse errors: Bad plan. You planned 176 tests but ran 184.
Files=5, Tests=36330, 18 wallclock secs ( 6.12 usr 0.03 sys + 3.86 cusr 0.20 csys = 10.21 CPU)
Result: FAIL
Failed 1/5 test programs. 8/36330 subtests failed.
make: *** [test_dynamic] Error 255



The tests for 'Cache::Memcached::Managed' failed. Would you like me to proceed anyway or should we abort?

Proceed anyway? [y/N]:
[ERROR] Unable to create a new distribution object for 'Cache::Memcached::Managed' -- cannot continue

*** Install log written to:
/home/zby/.cpanplus/install-logs/Cache-Memcached-Managed-0.20-1251915361.log

Error installing 'Cache::Memcached::Managed'
Problem installing one or more modules

zby@zby:~/progs/Enzyme$


Yeah - the information is there - but now imagine that this was a deep dependency tree with lot's of similar external prerequisites. You try to install the module - it fails, you view the log, locate the problem, install the external dependency, run the installation again and it fails again. Everytime you need to parse the really long installation log and try to understand what happened. It is not that complicated (although it can become complicated when there aren't so explicite error messages, or when there are other installation failures mixed together) - but it is intimidating and really time consuming. Definitively it is not something that a newcomer would be able to do. But external dependencies are not something extraordinary - they are rather standard, this procedure I described here is not exactly a failure mode - it is anticipated.

Someone could argue that when I install Cache::Memcached::Managed I should know that it needs the memcached binary - but it is only obvious for those that know that module, and if that module is somewhere deep in the dependency tree you might not even realize that you are installing it.

Now imagine this:

zby@zby:~/progs/Enzyme$ cpanp i Some::Module
Computing prerequisites.
You have unmet external prerequisites:
memcached
xslt version 1.1.18
...
Please install the prerequisites first then run the installation again.

wouldn't that be helpful?

PS. How about

requires_external memcached => sub { -x "memcached" }

in Makefile.PL?