A fix for this should be backported to 3.x as well.
The documented behavior is to return an object "with the entries for which the block evaluates to true" which describes the behavior for the second example. I will assume that the fix for this should change the Hash behavior. I.e. the first example should return an empty hash.
ok, then lets use boolean true as oppose to thruthy.
merged to 3.x here: 85fd467
1. Fixed versions (noted above) include 4.3.0, but this fix doesn't appear to be merged to master.
2. This function doesn't seem to work at all in 3.x, current version or versions before the fix date.
[root@k9xyz4acfmlsqq7 tmp]# puppet --version
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3.8.3
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[root@k9xyz4acfmlsqq7 tmp]# cat test.pp
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notice([1,2,3].filter |$x| {$x})
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[root@k9xyz4acfmlsqq7 tmp]# puppet apply test.pp
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Error: Could not parse for environment production: Syntax error at '.'; expected ')' at /tmp/test.pp:3 on node k9xyz4acfmlsqq7.delivery.puppetlabs.net
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Error: Could not parse for environment production: Syntax error at '.'; expected ')' at /tmp/test.pp:3 on node k9xyz4acfmlsqq7.delivery.puppetlabs.net
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[root@k9xyz4acfmlsqq7 tmp]#
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The hash version of the call fails in a similar way.
Thomas Hallgren, could you please check this out? Thanks.
You need to use --parser future on 3.x to use iterative functions, lambdas etc. 
afaict, it is merged to 3.x, stable, and master. This shows all branches where a commit is included (-r searches your remote branches; need to do pull/fethch first naturally).
git branch -r --contains 85fd467
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Using --parser=future for the 3.8.x tests. Thanks Henrik.
The arrays and the hashes behave in a consistent way now, the block must evaluate to TRUE and not just to a truthy value.
Is this what the customer will expect from reading the documentation of the filter function? (https://docs.puppetlabs.com/references/latest/function.html#filter) Should that document be worded more strongly to prevent misunderstanding? cc Jorie Tappa
Verified on redhat-7-x86_64:
3.8.3, before fix:
[root@swtuf7f3znrh2qz puppet]# puppet --version
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3.8.3
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[root@swtuf7f3znrh2qz puppet]# puppet apply -e 'notice({"a" => 44, "b" => true}.filter |$k, $v| {$v})' --parser=future
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Notice: Scope(Class[main]): {a => 44, b => true}
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Notice: Compiled catalog for swtuf7f3znrh2qz.delivery.puppetlabs.net in environment production in 0.45 seconds
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Notice: Finished catalog run in 0.06 seconds
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[root@swtuf7f3znrh2qz puppet]#
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3.8.4, after fix:
[root@p72k0s0obo5sgss puppet]# puppet --version
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3.8.4
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[root@p72k0s0obo5sgss puppet]# puppet apply -e 'notice({"a" => 44, "b" => true}.filter |$k, $v| {$v})' --parser=future
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Notice: Scope(Class[main]): {b => true}
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Notice: Compiled catalog for p72k0s0obo5sgss.delivery.puppetlabs.net in environment production in 0.47 seconds
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Notice: Finished catalog run in 0.01 seconds
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[root@p72k0s0obo5sgss puppet]#
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Also verified in the master branch:
[root@i7ykvf2snzu7y5g ~]# puppet --version
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4.2.3
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[root@i7ykvf2snzu7y5g ~]# puppet apply -e 'notice({"a" => 44, "b" => true}.filter |$k, $v| {$v})'
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Notice: Scope(Class[main]): {b => true}
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Notice: Compiled catalog for i7ykvf2snzu7y5g.delivery.puppetlabs.net in environment production in 0.37 seconds
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Notice: Applied catalog in 0.02 seconds
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[root@i7ykvf2snzu7y5g ~]#
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