Stream: beginners

Topic: Is there a better way to write this simple function?

Matthieu Pizenberg (May 09 2024 at 18:58):

I’m not looking for production-grade code, mainly is it idiomatic or is there more idiomatic way to write that function computing the Caesar Cipher of a string?

expect caesar "abc" 1 == Ok "bcd"
expect caesar "abc" 25 == Ok "zab"

caesar : Str, U8 -> Result Str _
caesar = \plaintext, shift ->
    Str.walkUtf8 plaintext [] \acc, letter ->
        List.append acc ((letter - 'a' + shift) % 26 + 'a')
    |> Str.fromUtf8

Hristo (May 09 2024 at 19:18):

I can't answer your question regarding whether the following may be more idiomatic or not, but if your expected input is only ASCII, you might consider also (in pseudocode):

1. Convert the string to List U8.
2. Split at a particular index, based on shift.
3. Concatenate the halves accordingly.
4. Join back into a string.

I suspect this version might be a tiny bit more readable (as you wouldn't need to do the element-wise shifting explicitly).

Matthieu Pizenberg (May 09 2024 at 19:31):

Indeed that’s more readable to convert to List U8 first, thanks @Hristo

caesar : Str, U8 -> Result Str _
caesar = \plaintext, shift ->
    Str.toUtf8 plaintext
    |> List.map \c -> (c - 'a' + shift) % 26 + 'a'
    |> Str.fromUtf8

Matthieu Pizenberg (May 09 2024 at 19:33):

I’m not sure what you meant by split and concat though

Brendan Hansknecht (May 09 2024 at 19:34):

The list version should also be a lot more performant

Brendan Hansknecht (May 09 2024 at 19:34):

It should happen inplace

Hristo (May 09 2024 at 20:20):

@Matthieu Pizenberg you're absolutely right!

I’m not sure what you meant by split and concat though

I wasn't being clear at all in my message above. But I'm also glad that - despite my confusing pseudocode - you arrived at a version which you feel is better.

I was thinking in terms of the Caesar cipher being an explicit mapping, like so:

alphabet = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z']

ceasar : Str, U8 -> Str
ceasar = \plaintext, shift ->
    mapChar = ceasarHelp shift
    Str.toUtf8 plaintext
    |> List.map mapChar
    |> Str.fromUtf8
    |> Result.withDefault ""

ceasarHelp : U8 -> (U8 -> U8)
ceasarHelp = \shift ->
    { before, others } = List.split alphabet (Num.toU64 shift)
    shifted = List.concat others before
    zipped = List.map2 alphabet shifted Pair
    dict = List.walk zipped (Dict.empty {}) \mapping, (Pair input output) ->
        Dict.insert mapping input output
    \input -> Dict.get dict input |> Result.withDefault 0

expect ceasar "abc" 1 == "bcd"
expect ceasar "abc" 25 == "zab"

Clearly, this isn't as readable - I have to admit that prior to writing it out, the mapping part seemed (in my mind) it'd be neater to implement and subsequently read.

Thanks @Brendan Hansknecht for jumping in to confirm regarding idiomaticity and efficiency!

Matthieu Pizenberg (May 09 2024 at 20:27):

Right! yes I was missing the alphabet part. I like that version too thanks for that. Not as terse but shows more of the language

Brendan Hansknecht (May 09 2024 at 20:38):

This last version will probably be a lot slower. Dict in the hot loop will be a lot slower than a little bit of math.

Hristo (May 09 2024 at 20:50):

Yeah, that's true! :100:
It might be more useful in cases where arbitrary mappings are needed (instead of something that could be computed arithmetically).

Matthieu Pizenberg (May 09 2024 at 21:03):

Right, that’s more useful in settings where the substitution key of the cipher is arbitrary

Last updated: Jul 26 2025 at 12:14 UTC