Anton

Posted on Sep 1, 2018

Elm challenge on Exercism: Rna Transcription

#elm #exercism #codechallenge

I'm gonna be short.

I'm going through exercism exercises.

At one moment I had a beef with the compiler.

The 4th and 5th branches of this if produce different types of values.

4| if x == 'G' then
5| 'C'
6| else if x == 'C' then
7| 'G'
8| else if x == 'T' then
9| 'A'
10| else if x == 'A' then
11| 'U'
12| else
13|> "There is an Unknown Character in the DNA sequence!!!"

The 4th branch has this type:

Char

But the 5th is:

String

Hint: All the branches of an if need to match so that no matter which one we
take, we get back the same type of value overall.

To me, it seemed unreasonable :)

Now, I'm just stuck here.

The definition of toRNA does not match its type annotation.

The type annotation for toRNA says it always returns:

Result Char String

But the returned value (shown above) is a:

String

Detected errors in 1 module.

Top comments (8)

DrBearhands • Sep 2 '18 • Edited

You have a fundamental representation problem:
you should not use a Char to describe a nucleotide base. Allowed values are only A, C, T and G. With this representation it could also be Z or ;...

Instead, create a new type:

type DNANucleotideBase
  = Adenine
  | Thymine
  | Guanine
  | Cytosine

or you can also use A | T | G | C for brevity if that does not conflict with anything.

That way you will not have to consider incorrect values because they cannot exist.

Also consider using a case x of rather than multiple ifs.

Anton • Sep 2 '18 • Edited

First, I'm gonna try deal wit the case and then with the Result type and then I will add types.

P.S. I just got my hand on frondendmasters elm courses. I will postpone solving the problem. Want to watch the videos first.

DrBearhands • Sep 2 '18

You won't need Result in this case if you make impossible state impossible :-)

BTW, have you tried the official Elm documentation? I'm not sure how they hold up for people new to functional programming but I myself found them sufficient (for the fundamentals anyway).

stevensonmt • Sep 2 '18

The exercise is written such that it expects to get a Result.
github.com/exercism/elm/blob/maste...

DrBearhands • Sep 2 '18 • Edited

I would argue this isn't a good exercise then. There are better way to explain result, that do not promote improper state representation (such as making a safe division function).

I apologize for being harsh. I understand writing this stuff is time-consuming and sometimes hard, but I do believe this (making impossible state impossible) is an important concept, central to Elm and FP in general.

stevensonmt • Sep 2 '18

Consider rewriting those conditionals as a case statement. Then make the return of the case statement a Result type.

case x of
    'G' -> Just 'C'
    'C' -> Just 'G'
    'T' -> Just 'A'
    'A' -> Just 'U'
    _ -> Err "There is an Unknown Character in the DNA sequence!!!"

Now all arms are the same type and the compiler won't complain.

Anton • Sep 2 '18

I wanted to use case, but then I wondered if elm has if else expression. So I didn't get to the case.

Thanks for pointing out to the Result type. I didn't know that exists.

stevensonmt • Sep 2 '18

I believe case statements are considered more idiomatic in elm.

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Elm challenge on Exercism: Rna Transcription

Top comments (8)

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