elm/parser

Regular expressions are quite confusing and difficult to use. This library provides a coherent alternative that handles more cases and produces clearer code.

(Readme.md from the package)

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

point : Parser Point
point =
  succeed Point
    |. symbol "("
    |. spaces
    |= float
    |. spaces
    |. symbol ","
    |. spaces
    |= float
    |. spaces
    |. symbol ")"

This is a summary of the talk Demystifying Parsers by Tereza Sokol.

Basics

Function	Description
run : Parser a -> String -> Result (List DeadEnd) a	Runs the Parser on a String
int : Parser Int	Parser for Int
float : Parser Float	Parser for Float
oneOf : List (Parser a) -> Parser a	Trys different Parsers. Uses the first that succeeds. Once the parser matches the first element, there is no going back!
andThen : (a -> Parser b) -> Parser a -> Parser b	Applys the first parser and if successful applies the second. Fails if one of the parsers fails.

Pipeline

This subject is explained at 4:48 in the Video.

Function	Description
succeed : a -> Parser a	Starts the Pipeline. a -> Parser a is the constructor. For a record Point use the structur called Point .
(|=) : Parser (a -> b) -> Parser a -> Parser b	Keeps an element.
(|.) : Parser keep -> Parser ignore -> Parser keep	Eats a character and throws it away.
spaces : Parser ()	Represents whitespace

Strings

This subject is explained at 7:20 in the Video.

Function	Description
chompWhile : (Char -> Bool) -> Parser ()	Looks for zero or more characters that succeeds the check. Stops as soon as the check fails.
chompIf : (Char -> Bool) -> Parser ()	Reads one symbols.
getChompedString : Parser a -> Parser String	Returnes the string that got chomped

Loops

This subject is explained at 14:10 in the Video.

Function	Description
loop : state -> (state -> Parser (Step state a)) -> Parser a	Takes an initial state and a parser step and returns a parser.
type Step state a = Loop state | Done a	A parser step just specifies if it should continue looping or stop.

Error messages

This subject is explained at 17:04 in the Video.

Function	Description
problem : String -> Parser a	Returns the error message and ends the parsing. The argument is the actuall message.

Further Reading

• 🎥Video: Demystifying Parsers by Tereza Sokol

• 📄Thread: Elm parser question