Differing outputs when FlatZinc code is passed via stdin vs. file

[chillja]

When running the following FlatZinc file minimal.fzn:

var 1..2:X_INTRODUCED_0_:: output_var;
var 1..2:X_INTRODUCED_1_:: output_var;
array[1..2] of var int:foobar:: output_array([1..2]) = [X_INTRODUCED_0_, X_INTRODUCED_1_];
constraint int_ne(X_INTRODUCED_0_, X_INTRODUCED_1_);
solve  satisfy;

with cat minimal.fzn | minizinc --input-from-stdin --solver Chuffed (or gecode) the output only prints the int vars:

X_INTRODUCED_0_ = 1;
X_INTRODUCED_1_ = 2;
----------

but when running minizinc --solver Chuffed minimal.fzn (or gecode) the output also correctly outputs the variable array:

X_INTRODUCED_0_ = 1;
X_INTRODUCED_1_ = 2;
foobar = array1d(1..2,[1, 2]);
----------

I am not (yet) familiar enough with the libminizinc code base to make any educated guess, where this might come from, or whether this is user error, but I think both should output the array.

[Dekker1]

The difference between the two commands is that when using --input-from-stdin the MiniZinc compiler will expect the input to be a MiniZinc model (.mzn).

In a MiniZinc model, FlatZinc annotations (output_var and output_array) are ignored. The default rules are then used to determine the output. In this case the variable are added, because they are decisions, but the array is not because it has a definition in its declaration.

As far as I know, there is currently no input from stdin that would allow you to pass in FlatZinc (as FlatZinc) to the compiler. However, some solvers do allow this.

[cyderize]

I believe there is an argument --input-is-flatzinc which can be used to inform MiniZinc that the input is FlatZinc (it seems to be currently undocumented however). Combining it with the --input-from-stdin flag should give the correct results.

[chillja]

Thanks a lot! If appreciated I'd like to add --input-is-flatzinc to the reference manual, as a remark on the --input-from-stdin option.

[Dekker1]

Feel free to send a pull request. The --help information is printed using the printHelp function found in flattener.cpp