Re: Ada can't initialize (aggregate) array of length 1?

[dewar@cs.nyu.edu (Robert Dewar)]

Scott is surprised that GNAT can give a clear error message for the case
of an invalid attempt to use a one-element positional aggregate.

Indeed this error message can't possibly be given in all situations (because
otherwise the language could allow it!)

Basically the cases that GNAT cannot correct are cases where you write
valid Ada 95 code that simply isn't quite what you meant:

    procedure x (a : integer);
    procedure x (a : array_of_integer);

now you write

    x ((1));

thinking that you are calling the second version with an aggregate, when
in fact you have written a valid call to the first version.

However, if there is an error, GNAT does the following:

  if I expect type vector of clunk
  and I actually have something of type clunk
  and the expression in question is parenthesized

  then output the appropriate error message complaining about misuse
  of 1-element positional aggregates.

of course no error message can always be "right". When a compiler detects
an error, it is in the business of producing the best guess as to what
was intended. The above check is just one of many heuristics in the GNAT
error detection intended to give more accurate messages.

By the way, whenever you get an error message you don't like from GNAT,
send a note to gnat-report. It is impossible to always give good error
messages, but often such suggestions are useful in tuning up the error
messages.