A simple PEGjs parser for a basic custom type system (Parses primitive types, generics (lowcase symbols), arrays (haskell way), functions (args are comma separated and after a right arrow there is a return type) and union types). Can be tried out here https://pegjs.org/online

basic-type-system-peg.js

// Copyright Antonio De Lucreziis 2018

/*

A simple PEGjs parser for a basic custom type system (Parses primitive types, generics (lowcase symbols), arrays (haskell way), functions (args are comma separated and after a right arrow there is a return type) and union types).

Can be tried out here https://pegjs.org/online

*/

Expression
	= symbolName:( Symbol _ "::" _)? symbolType:(Lambda / Type) {        
        if (!symbolName) {
       		return symbolType;
        }
        else {
        	return {
            	name: symbolName,
                type: symbolType,
            };
        }
    }

Type
	= UnionType / NonSetTypes

NonSetTypes
	= GenericParameter / PrimitiveType / ArrayType / ParenType

ParenType
	= "(" type:(Lambda / Type) ")" { return type; }

Lambda
	= args:( Type ( _ "," _ Type)* )? _ "->" _ ret:Type {
    	return {
        	is: 'lambda',
            'parameters': [args[0]].concat(args[1]),
            'return': ret,
        };
    }

UnionType
	= first:NonSetTypes rest:( _ "|" _ Type)+ { 
    	return {
        	is: 'union',
            types: [first].concat(rest.map(it => it[3]))
       	}; 
    }

ArrayType
	= "[" t:Type "]" { return { is: 'array', type: t }; }

GenericParameter
	= [a-z]+ { return { is: 'generic', name: text() }; }

PrimitiveType
	= [A-Z][a-zA-Z]* { return { is: 'primitive', name: text() }; }

Symbol
	= [^ ]+ { return text() }

_ "whitespace"
  = [ \t\n\r]*