Toy project: grammar rules in Haskell.

Example:

import Grammar

arithmeticGrammar =
    let number = OneOrMore $ Disjunction $ map (Literal . show) [0 .. 9]
	plus   = Sequence [number, Literal "+", expr]
	minus  = Sequence [number, Literal "-", expr]
	expr   = Disjunction [plus, minus, number]
    in  Grammar { start = expr }

matches arithmeticGrammar "4"           == True
matches arithmeticGrammar "1+2-32"      == True
matches arithmeticGrammar "text"        == False

See src/Grammar.hs for implementation. See test/Spec.hs for tests / example usage.

Note that this module does not return a parse tree; it returns only the boolean of whether the string can be generated by the grammar.

A more advanced example, with parenthetical expressions, optional whitespace, and scientific notation literals:

import Grammar

arithmeticGrammar =
    let whitespace = ZeroOrMore $ Literal " "
        number     = Sequence
            [ OneOrMore $ Disjunction $ map (Literal . show) [0 .. 9]
            -- optional scientific notation
            , Optional $ Sequence
                [ Disjunction [Literal "e", Literal "E"]
                , Optional $ Literal "-"
                , OneOrMore $ Disjunction $ map (Literal . show) [0 .. 9]
                ]
            ]
        term = Disjunction
            [ Sequence [whitespace, number, whitespace]
            , Sequence [whitespace, Literal "(", expr, Literal ")", whitespace]
            ]
        expr = Disjunction
            [ Sequence [term, Literal "+", expr]
            , Sequence [term, Literal "-", expr]
            , Sequence [term, Literal "*", expr]
            , Sequence [term, Literal "/", expr]
            , Sequence [term, Literal "^", expr]
            , term
            ]
    in  Grammar { start = expr }

matches arithmeticGrammar "15 * (3e5 + 17)"             == True
matches arithmeticGrammar "1 - 15 * (33e-5)"            == True
matches arithmeticGrammar "(1 - (15 * (33e-5)))"        == True
matches arithmeticGrammar "(32 ^ 5e5) + 2e7"            == True