[Haskell-cafe] Re: A Strict GCL Interpreter in Haskell

[Heinrich Apfelmus]

Tillmann Rendel wrote:
> 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.

And of course, these case expression are a case for the error monad. I
think that Wadler's paper is a great introduction to writing an
interpreter like the one at discussion here

  Monads for functional programming.
  Philip Wadler.
  http://homepages.inf.ed.ac.uk/wadler/papers/marktoberdorf/baastad.pdf


Regards,
apfelmus

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