Re: Peculiarity in Ada grammar

clsi!graham@uunet.UU.NET (Paul Graham) writes:


      Ada has only one precedence for minus, at the cost of giving unary minus
      lower precedence than multiplication operators. This has the unintuitive
      effect of making
-5 mod 3 = -(5 mod 3)
      while
-5 mod 3 /= (-5) mod 3


      I wonder if Ada's expression grammar was designed with operator precedence
      parsing in mind? Otherwise, how can the unintuitive precedence of unary
      minus be explained?


Probably with one word: "FORTRAN".


In Fortran,


            -5*3 = -(5*3)


Further, while FORTRAN 77 doesn't allow consecutive operators (i.e. an
expression like "5**-3" is nonconforming), some vendors that extend it to
do so end up doing things that also seem counterintuitive:


            5**-3*5 = 5**(-(3*5))


The above mystery is because, while ** (exponentation) is the
highest-precedence operator of the set {**, -, *}, it cannot be evaluated
until the unary minus operator (-) is evaluated. But that cannot be
evaluated before the multiplication (*) operator is evaluated, since the
latter is higher precedence than the former.


So perhaps, like Fortran, the designers of Ada viewed unary minus as just
shorthand for binary minus with a left-hand operand of 0 (of the
appropriate type).
--
James Craig Burley, Software Craftsperson burley@gnu.ai.mit.edu
--

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