Re: A new view of guards

[Simon L Peyton Jones]

| We can avoid both the case expressions and the helper function by Simon
| Peyton Jones' guard syntax
| 
|   -- version 3
|   simplify (Plus e e') | s  <- simplify e ,
|                        s' <- simplify e',
|                        (Val 0) <- s       
|                      = s'
|                        | s  <- simplify e ,
|                        s' <- simplify e',
|                        (Val 0) <- s' 
|                      = s
|                        | s  <- simplify e ,
|                        s' <- simplify e'
|                      = (Plus s s')
|   simplify e           = e

I agree with the general thrust of your message (though I was unable to
think of a good syntax for it), but in this particular case there's no
problem.  A where clause will do nicely:

   simplify (Plus e e') | (Val 0) <- s  = s'
                        | (Val 0) <- s' = s
                        | otherwise     = Plus s s'
                        where
                          s  = simplify e
                          s' = simplify e'

    simplify e          = e