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