Re: [Haskell-cafe] Eval of a syntax tree for reduction

28 Mar 2006

      Steve Downey wrote:
...
It makes eval1 a bit more complicated, and not as straightforward 
translation from the type system being described, though.
e.g reducing If looks more like
eval1 (TmIfExpr t1 t2 t3) =
    let t1' = eval1 t1
    in  case t1' of
            { Just t1'' -> Just $ TmIfExpr t1'' t2 t3
            ; Nothing -> Nothing
            }
You should use the fact that Maybe is a monad:

eval1 (TmIfExpr t1 t2 t3) =
   do t1' <- eval1 t1
      return $ TmIfExpr t1' t2 t3
...
and eval then looks like
eval t =
    let t' = eval1 t
    in case t' of
         { Just t'' -> eval t''
         ; Nothing -> t'
         }
(In the above, I suspect you need Nothing -> t, no prime.)

BTW, there's no need to use let here:

eval t = case eval1 t of Just t' -> eval t'
                          Nothing -> t

Re: [Haskell-cafe] Eval of a syntax tree for reduction

Antti-Juhani Kaijanaho