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Variable scopes #programming #haskell #perl #python

Variable scopes, helping programmers since the days of yore. Typos are easy to make:

$ cat fun.hs
main :: IO ()
main = do
  mapM_ (\old_key -> print old_key) [1..5]
  mapM_ (\new_key -> print old_key) [6..10]

Looks pretty innocent, right? But I missed something... can we get some help from the compiler, please?

$ ghc fun.hs
[1 of 1] Compiling Main             ( fun.hs, fun.o )

fun.hs:4:28: error: Variable not in scope: old_key
  |
4 |   mapM_ (\new_key -> print old_key) [6..10]
  |                            ^^^^^^^

Ah, yes, good point, thanks!

How about an interpreted language?

$ cat fun.pl 
#!/usr/bin/perl

use v5.12;

foreach my $old_key (1, 2, 3, 4, 5) {
    say $old_key;
}

foreach my $new_key (6, 7, 8, 9, 10) {
    say $old_key;
}

I made the same mistake again, silly me! Does the interpreter help?

$ perl fun.pl
Global symbol "$old_key" requires explicit package name (did you forget to declare "my $old_key"?) at fun.pl line 10.
Execution of fun.pl aborted due to compilation errors.

Thanks!

Let's try another language often recommended to beginners and students alike:

$ cat fun.py
#!/usr/bin/python3

for old_key in [1, 2, 3, 4, 5]:
    print(old_key)

for new_key in [6, 7, 8, 9, 10]:
    print(old_key)

Oops, I did it again! Hopefully the interpreter to the rescue:

$ python3 fun.py 
1
2
3
4
5
5
5
5
5
5

Oh no.