trjhtr

Here's my implementation of State Monad in TypeScript, based on a canonical Haskell implementation.
I would like it to get code reviewed.

class StateMonad<S, A> 
 constructor(public runState: (s: S) => ( s: S, a: A )) 
 

 static return_<S, A>(a: A): StateMonad<S, A> 
 return new StateMonad(s => ( s, a ));
 

 bind<B>(func: (a: A) => StateMonad<S, B>): StateMonad<S, B> 
 return new StateMonad<S, B>((s: S) => 
 const s: s_, a = this.runState(s);
 return func(a).runState(s_);
 );
 


// aux monad factory
const createCounter = (regex: RegExp) => new StateMonad((s: string) =>
 s.split('')
 .reduce((acc, c) =>
 (regex.test(c)) ? s: acc.s.replace(c, ''), a: acc.a + 1 : acc,
 s, a: 0 )
);

const countLowerCase = createCounter(/[a-z]/);
const countDigits = createCounter(/[0-9]/);


// usage example
const a = countLowerCase /* -- haskell equivalent */
 .bind(n1 => countDigits /* do n1 <- countLowerCase */
 .bind(n2 => StateMonad /* n2 <- countDigits */
 .return_(n1 + n2))) /* return n1 + n2 */
 .runState("abc123ABC");

I'm probably off my rocker here, but what if rather than nested chains of bind, you used a fork/join type of approach?

static combine<T, S, R>(
 monads: [P in keyof T]: StateMonad<S, T[P]> ,
 selector: (values: T) => R
)
 : StateMonad<S, R> 
 return new StateMonad<S, R>((state) => 
 var ret: any = ;
 for (const key in monads) 
 if (monads.hasOwnProperty(key)) 
 ret[key] = monads[key].runState(state).a;
 
 
 return a: selector(ret), s: state 
 
 );

and

var a = StateMonad
 .combine(
 
 countLowerCase,
 countDigits
 ,
 combined =>
 combined.countLowerCase +
 combined.countDigits
 )
 .runState("abc123ABC");