Hello comp.lang.vhdl.all,

When compiling the following test-code I encounter syntax errors, both in my
simulator and synthesis tool. This leads me to conclude that it involves a
language feature. The type involved (std_logic, bit, boolean) makes no
difference.

--      Error:
--      a<=b and c or d;

--      OK:
--      a<=(b and c )or d;

--      OK:
--      a<=b and (c or d);

Section 7.2 of the 1993 LRM defines the precedence of "and" and "or" as being
equal. The subsequent paragraph (p.92 line 55) specifies equal-precedence
operators must be evaluated in textual (left-to-right) order. However, the
sentence describing this textual order is qualified by the condition "Where
the language allows a sequence of operators". I don't find any obvious
definition of where such a sequence is allowed. The only paragraph coming
close can be found just before 7.2 (p.92 line 40) where I read:

NOTE- The syntax for an expression involving logical operators allows a
sequence of and, or, xor or xnor operators since the corresponding predefined
operators are associative.

I agree that not allowing operators with equal precedence to be intermixed
(esp. since "and" usually has greater precedence than "or" in Boolean algebra
notations) is a Good Thing. I just wanted to know:

* Is the aforementioned NOTE the cause of my error messages, or is there
another -perhaps clearer- clause tucked away somewhere in the LRM ?
* If the previous answer is "yes", isn't the sentence "sequence of and, or, or
xnor operators" ambiguous since it does not mention whether it involves a
sequence of the same operator or any of the operators mentioned?

I hope any of the language lawyers (Paul?) can elaborate on this.

Thanks in advance,

Hendrik De Vloed --  A4000/40 -- Aminet:hard/hack/lcdaemon.lha
-- Barco Silex FPGA/ASIC design   --   http://www.*-*-*.com/ --
***   #include<these_are_my_opinions_legal_mumbo_jumbo.h>  ***
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: When compiling the following test-code I encounter syntax errors, both in my
: simulator and synthesis tool. This leads me to conclude that it involves a
: language feature. The type involved (std_logic, bit, boolean) makes no
: difference.

: --      Error:
: --      a<=b and c or d;

: --      OK:
: --      a<=(b and c )or d;

: --      OK:
: --      a<=b and (c or d);

: Section 7.2 of the 1993 LRM defines the precedence of "and" and "or" as being
: equal. The subsequent paragraph (p.92 line 55) specifies equal-precedence
: operators must be evaluated in textual (left-to-right) order. However, the
: sentence describing this textual order is qualified by the condition "Where
: the language allows a sequence of operators". I don't find any obvious
: definition of where such a sequence is allowed. The only paragraph coming
: close can be found just before 7.2 (p.92 line 40) where I read:

: NOTE- The syntax for an expression involving logical operators allows a
: sequence of and, or, xor or xnor operators since the corresponding predefined
: operators are associative.

: I agree that not allowing operators with equal precedence to be intermixed
: (esp. since "and" usually has greater precedence than "or" in Boolean algebra
: notations) is a Good Thing. I just wanted to know:

: * Is the aforementioned NOTE the cause of my error messages, or is there
: another -perhaps clearer- clause tucked away somewhere in the LRM ?

If you carefully follow the EBNF in section 7.1, you will see that you
cannot freely intermix operators without the use of parentheses.
I.e., the BNF is *not*:

        expression ::= relation { logical_operator relation }

Instead, it is

        expression ::=
                  relation { _and_  relation }
                | relation { _or_   relation }
                | relation { _xor_  relation }
                | relation [ _nand_ relation ]
                | relation [ _not_  relation ]
                | relation { _xnor_ relation }

Notice that curly-braces are used around the associative operators,
thus allowing A and B and C.  However, square brackets are used around
non-associative operators, thus disallowing A nand B nand C.

The reason for all this?  Simple, (A and B) or C /= A and (B or C) and
(A nand B) nand C /= A nand (B nand C), and one of VHDL's guiding principles
is to prevent silly mistakes.

: * If the previous answer is "yes", isn't the sentence "sequence of and, or, or
: xnor operators" ambiguous since it does not mention whether it involves a
: sequence of the same operator or any of the operators mentioned?

No, it's not ambiguous, it's illegal.

Hope this helps,

Paul

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