Translated from C#. I'm new to F# so careful review recommended.

F# description tells to return Nothing if conversion is not possible. For F# it should read None instead of Nothing.

F# description talks about arrays, but tests expect a list.

Compared to other solutions here mine is overly verbose and has an unnecessary Seq conversion when Array.map exists.

Moved tests use statements to tests module. In the unlikely event of a user solution using rand::thread_rng or rand::seq::SliceRandom there will not be a double import with this change.

for Rust,

Description says that the input will only contain /[a-zA-Z0-9]/, however in the random test we generate random unicode characters. By writing println!("{}", text); to the start of your solution, you will see that the random tests generate emojis, chinese symbols and stuff that the console will print as blocks.

in test_codewarsisawersome we get string "CODEwarsIsAWesOmE" and expect there to be 5 duplicates.

My counts are:

no duplicates: cdrim      
duplicates:    oewas           <- count of 5

By this manual counting method I actually got 5 duplicates, just as the test expects.

Maybe I counted wrong or misunderstood the issue. If that was the case, please ignore this comment.

I made a C# solution that throws an ArgumentException when the town has a zero. The solution passes sample tests and actual tests.

I'm fairly confindent that in C# the town has no zeroes.

That does make more sense.

Some other languages (e.g. python, cpp, js) do allow index1 to be higher. Perhaps that inconsistency has nothing to do with the quoted snippet of description.

For Go

the description reads:

For the purposes of this kata, some tests may have multiple answers; any valid solutions will be accepted.

However, both actual and sample tests expect two_sum([1,2,3], 4)) to equal [0,2]. [2,0] is a valid solution for the inputs, but it fails the tests.

I don't think the Kata solver has to deal with test generated warnings in any manner at all, simply ignore them.

You can pass the Kata while having warnings.