the dreaded offside rule

Ben Rudiak-Gould Benjamin.Rudiak-Gould at
Thu Mar 9 21:00:56 EST 2006

Ian Lynagh wrote:
> On Thu, Mar 09, 2006 at 04:53:52PM -0000, Simon Marlow wrote:
>> the problem is that 'let' does not always have a
>> matching 'in', e.g. when it is used in 'do', pattern guards or list
>> comprehensions.  So you can't consistently treat let/in as brackets.
> Right, I mentioned that in my earlier mail. However, I think this can be
> handled by rules like
>      L (<n>:ts) ((Let:bs,m):bsms)   =   L ts ((bs,m):bsms)  if m = n

Here's a tricky case:

    ... | let x, y :: T
              x = 3
              y = 4,

The first , shouldn't close the let statement, but the second should. How do 
you distinguish between them? If you never allow , to close a let statement, 
then you can no longer write

    ... | let x = 3, ...

Of course, the commas in type signatures and fixity declarations are 
redundant anyway. If they were dropped from the grammar, this problem would 
go away.

> Incidentally, in my head the "," in "(case x of p -> e, 42)" acts as a
> right and left bracketing lexeme, so this expression would still be
> accepted.

Here's another tricky case:

    (case p of q | r,s -> t, ...)

I'm not sure what to do about this one.

-- Ben

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