Grammar Templates for Expressions?

[DavePearce]

Currently, the grammar used for describing Whiley statements and expressions is imprecise in certain ways. For example when describing the syntax for an if statement, the condition is defined as a generic Expr, when in fact it can only be a boolean expression.

The question is whether or not we can describe this and, if so, do we want to?

[DavePearce]

One issue here is that making it a grammatical issue will affect what kind of error message is reported. For example, consider these two messages:

error: unexpected token encountered
if (1,2):
     ^

versus:

error: expected type bool, found (int,int)
if (1,2):
   ^^^^^

The former represents a grammatical error, whilst the latter represents a type error.

[DavePearce]

One approach here would be to mix type and grammatical rules together. This might not be as crazy as it sounds, provided it was described properly.