evaluate package:express

O(n). Just the value of an expression when possible (correct type), Nothing otherwise. This does not catch errors from undefined Dynamic values.

> let one = val (1 :: Int)
> let bee = val 'b'
> let negateE = value "negate" (negate :: Int -> Int)

> evaluate one :: Maybe Int
Just 1

> evaluate one :: Maybe Char
Nothing

> evaluate bee :: Maybe Int
Nothing

> evaluate bee :: Maybe Char
Just 'b'

> evaluate $ negateE :$ one :: Maybe Int
Just (-1)

> evaluate $ negateE :$ bee :: Maybe Int
Nothing