🚀 go-pugleaf

I am interested in people's opinions on the restrictions of compatibility
between a FOR loop control variable and the types of limit expressions.
On page 158, section 9.8, of Wirth's 3rd, corrected ed. of Programming in
Modula-2, he states that the limit expressions (he calls them the starting
value and limit) must be "compatible" (as distinct from "assignment
compatible") with the control variable.  On page 65, he says

        "Two types are are said to be compatible, if they are declared
        as equal or satisfy certain compatibility rules to be discussed
        subsequently.  An important case of exceptional compatibility
        are the types INTEGER and CARDINAL.  Hence, the assignment
        i := c is admissible."

Finally, on page 148, section 6.3, he says

        "A type T1 is said to be compatible with a type T0, if it is
        declared either as T1 = T0 or as a subrange of T0, or if T0 is
        a subrange of T1, or if T0 and T1 are both subranges of the
        same (base) type."

At least one text book on Modula-2 says that the limit expressions must
be "assignment compatible" with the control variable (see p. 103 of
"Modula-2 Discipline & Design" by Arthur Sale, Addison-Wesley, 1986).
The issue arose during my work to port the 1-pass MC68K native code ETH
compiler.  The parser appears to define yet another type of compatibility.
It allows the starting value to be truely "assignment compatible".
However, the limit must be either 1) equal type with the control variable
or, 2) a CARDINAL constant < MAX(INTEGER) when the control variable is an
INTEGER type.  In other words, it does NOT allow a CARDINAL variable limit
with an INTEGER control variable, a CARDINAL constant limit with a CARDINAL
control variable, a CARDINAL variable limit with a CARDINAL control
variable, etc.  I intend to change it to allow a free mix of CARDINAL
and INTEGER control variable and limit expressions, but I wanted to get
others opinions and interpretations first.  Have the standardization
committees dealt with this?
______________________________________
Harvard Townsend, Systems Manager
Dept. of Computing & Information Sciences
Kansas State University, Manhattan, KS 66506   (913)532-6350
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