use variable to index into array of records with arrays

[Andrey Rybalchenko]

Hi, 

Any plans to lift this restriction, is it fundamental?

Error: type error: array access using a variable is not supported for array of a record type which contain an array.

My current workaround is to use 2d arrays, which are jagged so require additional bookkeeping.

Thanks,

Andrey

[Jip Dekker]

Hi Andrey,

The problem is fundamental in the sense that it has the same problem as your workaround. Because the arrays in each of the records can have different index set, selecting one of the records with a variable leaves us in a position were we cannot determine an index set. This goes against a lot of the assumptions within the MiniZinc compiler, and would currently make it impossible to compile array access for these arrays correctly according to MiniZinc's relational semantics.

With new compilation techniques that can take "real" variable length arrays, it might be possible to change this in the future. However, I think this is currently not a priority, and I don't see this changing soon. Instead using a 2d array with using optional types would seem the right solution for most of these situations.

Cheers,

Jip

[Andrey Rybalchenko]

Hi Jip,

Thanks for the clarification. 

Andrey

PS: It looks like even if the nested array is of fixed length, the compiler is not taking advantage of that.

---8<---

type Test = record(

  array [1 .. 4] of opt int: data,
  1 .. 4: size,
);

array [_] of Test: tests = [(data: [1,2,3, <>], size: 3), (data: [1,2, <>, <>], size: 2)];
var 1 .. card(index_set(tests)): x;

constraint tests[x].size = 2;

---8<---

Error: type error: array access using a variable is not supported for array of a record type which contain an array.

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[Jip Dekker]

Hi Andrey,

You're right that we probably could easily allow this when the arrays are guaranteed to have the same index sets. However, the problem is that index sets in MiniZinc are only guaranteed to be available at runtime. For now the check whether variable indexing for records is allowed is implemented in the typechecker, which does not yet have this information.

Cheers,

Jip