Auto-Delimited Expressions¶
Expressions appear everywhere to parameterize actions (and actions may appear in expressions, cf. Action As Expression). This may cause some syntax problems. For example, when writing:
print @f (1)
there is a possible ambiguity: it can be interpreted as the message with
two arguments (the function @f and the integer
1) or it can be the message with only one argument (the
result of the application of function @f to the argument
1). This kind of ambiguity appears in other places, as
for example in the specification of the list of breakpoints in a curve.
The cause of the ambiguity is that there is no separator between the
arguments of a messages. So we don’t know where the expression starting
by @f finishes.
The example here shows a more general problem which leads us to distinguish a subset of expressions: auto-delimited expressions are meaningful expressions that cannot be “extended” with what follows. Integers, for example, are auto-delimited expressions. We can write
print 1 (2)
without ambiguity: this is the message with two arguments because there is no other possible interpretation. Variables are another example of auto-delimited expressions.
Being auto-delimited is a sophisticated property involving the type of the actual value of the expressions. So, the Antescofo approach is to accept a simple syntactic subset of expressions to avoid possible ambiguities in the places where this is needed. This subset is defined by the syntax diagrams given above.
For example:
$x + 3 print BAD // syntax error: x + 3 is not auto-delimited
($x + 3) print OK // parenthetized expressions are always auto-delimited
To disambiguate our first example, we can also use parentheses:
print (@f (1)) // is interpreted as the print of one argument: (@f(1))
print @f (1) // is interpreted as the print of one argument: (@f(1))
print (@f) (1) // is interpreted as the print of two arguments: @f and 1
The second form is interpreted as only one argument, because a functional constant is useless as an argument in a message sent to the environment. But the third form shows how to force the alternative interpretation.