README

guard, when, and unless allow for conditional operations and short-cutting in monadic expressions.

Guards

Guards are used to stop the monadic expression continuing if a flag is true (for guard) or false (for guardnot).

They only work with monads that have an 'alternative value' (which is usually used as the error condition: Left in Either for example). An alternative value is provided when the guard triggers:

from x in ma
from _ in guard(x == 100, Error.New("x should be 100"))
select x;

Supported monads are:

Either
EitherUnsafe
EitherAsync
Fin
Validation
Aff
Eff

When and Unless

when and unless are similar to guards, but instead of providing the alternative value, you provide an alternative monad to run. This monad could be in a failed state, or it could run a successful side effect (an Eff calling Console<RT>.writeLine() for example).

from x in ma
from _ in when(x == 100, Console.writeLine<RT>("x is 100, finally!"))
select x;