Parsing Example in Elm

John_Orford 2018-07-24 10:42:19 UTC #1

Anyone willing to give some feedback on my simple parsing kata?

Perhaps make it stricter? Using an ADT is an obvious one…

john-orford/romans/blob/master/Main.elm

module Main exposing (..)

import Parser exposing (..)
        
i = keep (Exactly 1) (\c -> c == 'I')
    
v = keep (Exactly 1) (\c -> c == 'V')
      
x = keep (Exactly 1) (\c -> c == 'X')
    
l = keep (Exactly 1) (\c -> c == 'L')
    
c = keep (Exactly 1) (\c -> c == 'C')
    
d = keep (Exactly 1) (\c -> c == 'D')
    
m = keep (Exactly 1) (\c -> c == 'M')
    
iv = succeed (++)
     |= i

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There’s non-parsing code which can obviously be improved, but I am into parsers right now…

utkarshkukreti 2018-07-24 15:27:23 UTC #2

You can use symbol or keyword (depending on what kind of error message you want on failure) to parse a string at once instead of parsing it character by character. Here’s how I would rewrite the parser while keeping the rest of the code unchanged:

parse =
    "IV IX XL XC CD CM I V X L C D M"
        |> String.split " "
        |> List.map (\string -> succeed (always string) |= symbol string)
        |> Parser.oneOf
        |> repeat oneOrMore

John_Orford 2018-07-24 16:06:03 UTC #3

Oh, that’s nice - using symbol & keyword with succeed like that.

What I wanted to do was also return ADTs, rather than strings, just helps type check everything…

utkarshkukreti 2018-07-24 17:04:50 UTC #4

Here’s how you can return ADTs:

parse =
    [ IV, IX, XL, XC, CD, CM, I, V, X, L, C, D, M ]
        |> List.map (\rn -> succeed (always rn) |= symbol (toString rn))
        |> Parser.oneOf
        |> repeat oneOrMore

You just need to remove the quotes around the strings in the sum function now and it should work!

system 2018-08-03 17:04:52 UTC #5

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