funerl

What?

Maybe and either monads interfaces based on similar modules in Haskell.

Why?

Just for fun. It was an attempt to solve nested cases "code smell" or "antipattern" in Erlang.

Should I use this library?

Definitely not, just pass on.

Maybe monad

1> Val = maybe:just(1).
{ok,1}
2> NoVal = maybe:nothing().
undefined
3> maybe:map(fun(X) -> X+1 end, Val).
{ok,2}
4> maybe:map(fun(X) -> X+1 end, NoVal).
undefined

Either monad

1> Result = either:right(23).
{ok,23}
2> Error = either:left(badarg).
{error,badarg}
3> either:map(fun(X) -> X * 2 end, Result).
{ok,46}
4> either:map(fun(X) -> X * 2 end, Error).
{error,badarg}

You can use these monads to handle errors.

Using either:map/2 and either:bind/2 to prevent nested case expressions:

-spec request_order() -> either:either(term()).
request_order(Order) ->
    ...
    either:right(NewOrder).
    
-spec validate_order(Order :: term()) -> either:either(term()).
validate_order(Order) ->
    either:left(order_invalid).

-spec db_write(atom(), Order :: term()) -> term().
db_write(Table, Value) when is_atom(Table) ->
    ...
    Result = db:insert(Table, Value),
    ...
    Result;


-spec add_order(term()) -> either:either().
add_order(Order) ->
    Res = request_order(Order),
    MaybeNewOrder = either:bind(fun validate_order/1, Res),

    WriteFun = fun(Ord) ->
        db_write(order, Ord)
    end,
    either:map(WriteFun, MaybeNewOrder).

Returns {error, order_invalid}:

1> Order = ...
2> Result = add_order(Order).
{error, order_invalid}
3> either:is_left(Result).
true
4> either:is_right(Result).
false