nucleotide-count: Change types to return an Either.

[rbasso]

We are considering changing the interface of the problem nucleotide-count. The idea (#115) is to allow a more idiomatic solution to this problem, returning a Maybe, an Either or something else, instead of forcing the user to throw an error with the specific message: invalid nucleotide 'X'.

It could be something like this:

module DNA (count, nucleotideCounts) where

count :: Char -> String -> Maybe Int
nucleotideCounts :: String -> Maybe (Map Char Int)

This is just and sketch, and it can/should change. It depends only on the feedback here!

Any suggestion or comment is welcome! 😄

[abo64]

+1 for not not throwing any unforced errors in Haskell.
But if you change the signature I would rather go for an Either String (Map Char Int).
This way you also stay closer to the original exercise.

[rbasso]

In that case, what would Left String contain? An error message, the input String or just the invalid characters?

[abo64]

Definitely the error message. I think this is also how most people use Either for error handling.

[rbasso]

We don't need specify that. For the tests it probably would be enough to check if it is a Left.

[abo64]

fine with me.

[petertseng]

I'm assuming that if we'll have nucleotideCounts :: String -> Either String (Map Char Int) then we would also have count :: Char -> String -> Either String Int.

That seems reasonable to me (the initial suggestions of just the Maybe seem fine as well). Agreed that there's no need to test for a specific error message, just that it's Left.

[samjonester]

I agree with @abo64 that the original error string should be the Left. It feels like it would be an extremely useful message if I was to use these methods IRL.

[rbasso]

If anyone is willing to write a PR, I will gladly review/merge it. 😄

Anyone?