xBase gotchas

For the last year, I have regularly worked on a legacy project written in xBase / Clipper, which is a dialect of the dBase programming language. The language as a whole is surprisingly comfortable for its age, being 40 years old now. But there were a few quite infuriating things I stumbled upon:

Trailing commas in arrays

The language allows you to define array contents inline like this:

myArray := { 1, "foo", .t., 4.0 }

It is also dynamically typed, as you can see in this example – where the array holds numbers, strings and boolean (logical) values. Now sometimes, you want to spread-out an array initialization over multiple lines. xBase lets you do that – using the semi-colon quite contrary to many other languages – to continue a line:

myArray := {;
  "foo",;
  "bar";
}

Now you might be temped, as I was, to write it like this instead:

myArray := {;
  "foo",;
  "bar",;
}

With a trailing comma. But, gotcha, that does not do what you probably expect. This actually creates an array with 3 elements, where the last element is nil.

Unequal is not not-equals

xBase has both the == and the != operators. But imagine my suprise when and if a != b was not entered, with what had to be unequal strings. I added some logging (yea, no debugger) and saw that I indeed had the strings "" and "foo". Still, a != b seemed to evaluate to false. Because I could not believe that, I changed it to !(a == b) – and it worked!

It turns out that the != operator is not the opposite of the == operator, but rather of the = operator (xBase uses := for assignment). The = operator, however, implements some kind of “fuzzy” equals by default, also evaluating true if the left operand is a prefix of the right operand. This is a behaviour that can be changed globally by “SET EXACT on”. Wow.

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