Re: A Strict GCL Interpreter in Haskell

> Now we can write our interpreter:
>
>   eval :: Expression -> Result
>   eval (Literal n)
>     = Number n
>
>   eval (Add a b)
>     = case eval a of
>         Number result_of_a
>           -> case b of
>                Number result_of_b
>                  -> Number (result_of_a + result_of_b)
>
>                DivideByZeroError
>                  -> DivideByZeroError
>
>         DivideByZeroError
>           -> DivideByZeroError
>
>
>   eval (Add a b)
>     = case eval a of
>         Number result_of_a
>           -> case b of
>                Number result_of_b
>                  -> if b == 0
>                       then DivideByZeroError
>                       else Number (result_of_a + result_of_b)
>
>                DivideByZeroError
>                  -> DivideByZeroError
>
>         DivideByZeroError
>           -> DivideByZeroError
>
>
> This interpreter will stop as soon as it encounters a division by zero,
> because we take care to return DivideByZeroError whenever we see a
> DivideByZeroError in one of the subterms.
>
> So you have to use an appropriate return type (like (Either String Int)
> or Result), and you have to pattern match on the result of "earlier"
> parts of your program, and propagate the error.