Creates an error message from an Error data.

import Parser exposing (letter, parse)


-- Getting a digit instead of a letter.
parse "123" letter
    |> Result.mapError message
--> Err "1:1: I was expecting a letter [a-zA-Z].\nI got stuck when I got the character '1'."


-- Running out of input characters.
parse "" letter
    |> Result.mapError message
--> Err "1:0: I was expecting a letter [a-zA-Z].\nI reached the end of the input text."

Dumps the error into a human-readable format.

import Parser exposing (Parser, andThen, char, drop, into, parse, spaces, succeed, take)
import Parser.Common exposing (number)


spaces
    |> andThen (\_ -> number)
    |> parse "  abc  "
    |> Result.mapError (dump "filename.txt")
--> Err
-->     [ "[ERROR] filename.txt:1:3: I was expecting a digit [0-9].\n"
-->         ++ "I got stuck when I got the character 'a'."
-->     , ""
-->     , "1|  abc  "
-->     , "    ^"
-->     ]




type alias Point =
    { x : Float
    , y : Float
    }


point : Parser Point
point =
    into "Point"
        (succeed Point
            |> drop (char '(')
            |> take number
            |> drop (char ',')
            |> take number
            |> drop (char ')')
        )


spaces
    |> andThen (\_ -> point)
    |> parse "  (12,)  "
    |> Result.mapError (dump "filename.txt")
--> Err
-->     [ "[ERROR] filename.txt:1:7: I was expecting a digit [0-9].\n"
-->         ++ "I got stuck when I got the character ')'."
-->     , "  in Point at line 1:3"
-->     , ""
-->     , "1|  (12,)  "
-->     , "    ~~~~^"
-->     ]


type alias Line =
    { p1 : Point
    , p2 : Point
    }


line : Parser Line
line =
    into "Line"
        (succeed Line
            |> drop (char '[')
            |> take point
            |> drop (char ',')
            |> take point
            |> drop (char ']')
        )


spaces
    |> andThen (\_ -> line)
    |> parse "  [(12,34),(56,)]  "
    |> Result.mapError (dump "filename.txt")
--> Err
-->     [ "[ERROR] filename.txt:1:16: I was expecting a digit [0-9].\n"
-->         ++ "I got stuck when I got the character ')'."
-->     , "  in Point at line 1:12"
-->     , "  in Line at line 1:3"
-->     , ""
-->     , "1|  [(12,34),(56,)]  "
-->     , "             ~~~~^"
-->     ]

Dumps a snippet of the input text that caused the parser to fail.

import Parser exposing (Parser, andThen, char, drop, into, parse, spaces, succeed, take)
import Parser.Common exposing (number)


type alias Point =
    { x : Float
    , y : Float
    }


point : Parser Point
point =
    into "Point"
        (succeed Point
            |> drop (char '(')
            |> drop spaces
            |> take number
            |> drop spaces
            |> drop (char ',')
            |> drop spaces
            |> take number
            |> drop spaces
            |> drop (char ')')
        )


spaces
    |> andThen (\_ -> point)
    |> parse "  (12,)  "
    |> Result.mapError dumpCodeSnippet
--> Err
-->     [ "1|  (12,)  "
-->     , "    ~~~~^"
-->     ]


spaces
    |> andThen (\_ -> point)
    |> parse
        (String.join "\n"
            [ "  "
            , "  (  "
            , "  12  "
            , "  ,  "
            , "  )  "
            , "  "
            ]
        )
    |> Result.mapError dumpCodeSnippet
--> Err
-->     [ "2|  (  "
-->     , "3|  12  "
-->     , "4|  ,  "
-->     , "5|  )  "
-->     , " +~~^"
-->     ]