Re: Integral types and own type definitions (was Re: Suggested method for returning a string from a C program?)

On 26/03/2025 23:15, Keith Thompson wrote:

Janis Papanagnou <janis_papanagnou+ng@hotmail.com> writes:

On 26.03.2025 20:38, Keith Thompson wrote:

[...]

If -5 were a single token (an integer constant), then 6-5 would be an
integer constant 6 followed by an integer constant -5, a syntax error.

>
This is generally not true; that depends on the syntax rules. It is
perfectly fine to have rules for 2-ary addition and subtraction and
have also negative (and positive) literals or expressions with unary
plus and minus operators.
>
Disclaimer: As usual I'm not speaking about any peculiarity that "C"
may (or may not) have defined in its standard, but generally.

 I suppose it would be possible to define a language syntax that makes
-5 a single token and still parses 6-5 as (literal 6, binary-minus,
literal-5). 

I'm sure a language could have syntax and parsing rules that interpreted the minus sign as binary minus in "x-y", unary minus in "-x", and part of the literal in "-5" if it is not preceded by an operand that has a suitable subtraction operation.  But I think the rules would get fairly complicated - much more so than the rules of C syntax and parsing.
It would be a lot easier in a language that required spaces to separate tokens.  Then it would be simple to distinguish "x - y" and "-5" - the binary subtraction operator is always space-minus-space, while in a literal, the minus sign has no space on its right.  "- 5" would then be parsed as unary minus applied to the literal "5", and "-x" would be a single token unrelated to "x".  This is how it is handled in Forth - there, "-5" is a single token for the literal value of minus five (unless some silly bugger has redefined -5 to something else...).  If you want to apply unary minus to 5, you write "5 NEGATE" (as Forth uses RPN notation).

 I don't know of any language that uses "-" for both negation (prefix,
one operand) and subtraction (infix, two operands) and treats -5
as a single token rather than a unary minus operator applied to the
constant/literal "5".
 

For the most part, the important part of the grammar is the "constant expression" or "integer constant expression" - and you get that whether "-5" is parsed as one token (in a hypothetical C variant) or two tokens.