f should be called.

GiacomoSorbi, what do you mean?
It would be helpfull to get the input. I am with the same difficulty.
What is this "Haskell translator comment?"

This is expected. Hint: be as lazy as possible.

Now I get:

Hard properties

GNU MP: Cannot allocate memory (size=109117456)

Process was terminated. It took longer than 12000ms to complete

...

Is it better now?

Yes, it is meant to be (7 ^ (6 ^ 21)).

But is it meant to be (7 ^ (6 ^ 21)) at least?

No, because (7 ^ (6 ^ 21)) mod 10 is not equal to (7 ^ ((6 ^ 21) mod 10)) mod 10.

Same problem, please fix it, been stuck here twice...

Haskell version, tab character problem! Please fix!

should work for random strings
✘ Falsifiable (after 7 tests and 3 shrinks):
" "

I have no idea what is this. " " in, " " out, test failed. No reason given.

Cheating: First argument should be used

What does this message mean?

Where is Text.Parse coming from? Couldn't find it on official document. API looks like Text.Parsec though.

Yes, and "\201", "\213", etc. are considered uppercase. They really shouldn't be.