[Haskell-beginners] substitution

[Stephen Tetley]

Hi John

Your evalStep case for binary operations (Add Mult, etc) are only
going down the 'left-spine' when they don't match a Val

eg:

evalStep d(Add x y)
  = case x of
      (Val a) -> case y of
                   (Val b) ->  Val (a+b)
                   left -> Add x (evalStep d y)
      right -> Add (evalStep d x)y

The code wants to be something like this...


evalStep d(Add x y)
  = case x of
      (Val a) -> case y of
                   (Val b) ->  Val (a+b)
                   left -> Add x (evalStep d y)
      right -> Add (evalStep d x) (evalStep d y)
                                     ^^^^^^^^^


Best wishes

Stephen


On 1 March 2010 22:17, John Moore <john.moore54 at gmail.com> wrote:
> Hi,
>      Can anyone explain what I'm doing wrong in the subtitution of the Let
> statement. If you look at test three. It wont substitute the second var "x".
> I enclose the file because this program is getting too big to copy and
> paste.
>
> John
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