Ruby

#puts returns nil, so when you're mapping you end up with [nil, nil, nil], which prints with #puts as three blank lines. If you use p instead of puts inside the block, the block will return i instead of nil and you'll see the same output.

I think this is because do ... end blocks has lower precedence than {...} blocks. As documented in [1]. So, probably, not a bug.

method1 method2 { ... } # method2's block method1 method2 do ... end # method1's block 

[1] http://ruby-doc.org/core-2.5.1/doc/syntax/calling_methods_rdoc.html#label-Block+Argument