Tuesday, March 11, 2008

Poor Ada Error Message

I have been demonstrating on the York University "Algorithms and Data Structures" course for 4 years now. As part of the course, first year students learn Ada. A lot of the error messages are really bad - but over time I've created a mental mapping between the message and the cause. I am now fairly fluent at recognising what mistake a student has made, given the exercise they are attempting and the error message. But yesterday I encountered a brand new misleading error message.

The error message was:

bad.adb:12:22: actual for "N" must be a variable

As always, the most useful thing in the error message is the line number. I read enough of the error message to check whether its a parse error, type error or something else, then head for the line mentioned. (I follow this same tactic in all languages, not just Ada.)

12: Add_Cell(I, Next(N));

Nothing obviously wrong about this statement, so I read the error message. It seems to want N to be a variable. But I already know that N is a variable, or at the very least a parameter, so this condition seems to be met.

Next step is to head to the definitions of Next and Add_Cell, to see if they can shed some light on the situation.

function Next(N: List) return List is ...
procedure Add_Cell(I: Integer, N: in out List) is ...

The in out in Add_Cell can be read as "pass as a pointer". Aha, maybe the error message is complaining that the second argument to Add_Cell can't be made a pointer, as its a return value from a function. That would explain it, and indeed, that turned out to be the cause of the problem. But back to the error message, what was it trying to tell us?

bad.adb:12:22: actual for "N" must be a variable
12: Add_Cell(I, Next(N));

When the error message refers to N it isn't talking about the variable N I can see, but the second argument of Add_Cell, which is also called N. If the function being calling was in a separate library, it would have been even harder to understand. A more helpful error message might have been:

bad.adb:12:22: expression passed as the second argument to Add_Cell must be a variable
Found: Next(N)
Expected: A variable
Reason: Second argument of Add_Cell is declared "in out"

In general compiler error messages should be in terms of the line where the error resides, not requiring a large amount of global knowledge. The error can be resolved, but without help from the message. All compilers have bad error messages in some circumstances, but this one seems almost malicious!

1 comment:

Paul R. Potts said...

I think the clue is in the phrase "actual parameter." In Pascal, there are "formal parameters" (in the function or procedure definition) and "actual parameters" (at the point of call). Ada seems to have adopted this wording. It seems to be a fairly obsolete terminology.