Coding Challenge #2! (115)

107 Name: #!/usr/bin/anonymous : 2008-04-27 05:20 ID:xxFWOzRb

Monads in Javascript

// Monad stuff
function bindM(m ,k)
{
return function (s) {
tmp = m(s);
a = tmp[0]; s_ = tmp[1];
return k(a)(s_);
}
}
function thenM(m,k) { return bindM(m,function(_){return k;}); }
function returnM(v) { return function (s) { return [v,s]; } }
function evalS(m,s) { return m(s)[0]; }
function getS(s) { return [s,s]; }
function putS(s) { return function(_) { return [null,s]; } }
function doM(m) {
if (arguments.length == 1) return m;
else if (arguments.length % 2 && arguments.length >= 2) {
var rest = Array.prototype.slice.call(arguments).slice(2);
return arguments[1](m, doM.apply(this, rest));
}
else throw ("doM: Error: Arguments mismatch.");
}
function mapM(k,l) {
if (l.length > 0) return doM(
k(l[0])
, bindM
, function (item) { return doM(
mapM(k,l.slice(1))
, bindM
, function (rest) {
if (rest)
return returnM(item.concat(rest));
else
return returnM(item);
}
); }
);
else
return returnM(null);
}
// Parse stuff
function parse(parser, input) { return evalS(parser, input); }
function throwInt(msg,input) { 
var e = new Error();
e.message = [msg,input];
e.name = 'PARSE_ERROR'; throw e;
}
/// Combinators
function choice_(m,k) {
return function (s) {
try {
return m(s);
}
catch (e) {
switch (e.name) {
case 'PARSE_ERROR':
if (e.message[1].length != s.length) throw e;
else return k(s);
break;
default:
throw e;
}
}
}
}
function try_(m) {
return function (s) {
try {
return m(s);
}
catch (e) {
switch (e.name) {
case 'PARSE_ERROR':
e.message[1] = s; // Pretend we haven't consumed input
throw e;
break;
default:
throw e;
}
}
}
}
var letter_ = doM( 
getS
, bindM
, function (input) {
if (input.length == 0)
return throwInt("Unexpected end of input, expected letter",input);
if (input.match(/^[a-z]/i))
return doM( putS(input.slice(1)) ,thenM, returnM(input[0]) );
else
return throwInt("Expected letter ([a-zA-Z])",input);
}
);
function char_(c) {
return doM(
getS
, bindM
, function (input) {
if (input.length == 0)
return throwInt("Unexpected end of input, expected '" + c + "'",input);
if (input[0] == c)
return doM( putS(input.slice(1)) ,thenM, returnM(c) );
else
return throwInt("Expected character '" + c + "'",input);
}
);
}
function string_(str) {
return mapM(char_,str);
}
// Run a parser
function run(parser,string) {
print("Parse output:");
try {
print(parse(parser,string));
}
catch (e) {
switch (e.name) {
case 'PARSE_ERROR':
print("Parse failed: " + e.message[0] + ", input: " + e.message[1]);
break;
default:
throw e;
}
}
}
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