How do I return a value from a PureScript function

问题:

I've just started learning about PureScript effects, and I'm stuck trying to make a function that has an EXCEPTION effect.

lengthGt5 :: forall eff. String -> Eff (err :: EXCEPTION | eff) String
lengthGt5 a = if (length a <= 5)
              then throwException $ error "Word is not the right length!"
              else a

main = do
  word <- catchException handleShortWord (lengthGt5 "test")
  log word

  where
    handleShortWord err = do
      log (message err)
      return "Defaut::casserole"

When I try and run this I get the following error

Could not match type

    String

  with type

    Eff
      ( err :: EXCEPTION
      | eff0
      )
      String

I understand that lengthGt5 needs to return a String wrapped in an Eff in the non-exception case, but I'm not sure how I can create an "empty effect wrapper" around the value a. Am I thinking about this right?

回答1:

I figured out what I was missing. To return the value in the non-exception case, you have to call pure a

lengthGt5 :: forall eff. String -> Eff (err :: EXCEPTION | eff) String
lengthGt5 a = if (length a <= 5)
              then throwException $ error "Word is not the right length!"
              else (pure a)

pure is defined in the Applicative type class defined as follows:

class (Apply f) <= Applicative f where
    pure :: forall a. a -> f a

Applicative is a subclass of Apply and defines the pure function. pure takes a value and returns a value whose type has been wrapped with the type constructor f.

So pure takes the value a, and returns that value wrapped in a type constructor - in this situation the type constructor is Eff e