Syntax Proposals / Foreach loop

Is your feature request related to a problem? Please describe. When importing arrays, we need to know while writing the model the size of the imported array, as it is needed to generate the i of the for loop. Something this can be quite inconvenient.

Describe the solution you'd like We could extend the loop mechanism to authorize "foreach" behaviour:

#PARAMETERS
   x = import "long_file_with_unknown_length.csv";
//...
#CONSTRAINTS
   a == sum(v for v in x)

Semantically, we have multiple option:

• We can, as above, say that when we do for v in x, v we take each value contained in x. This is the behavior in most languages. It's quite natural for arrays (#9).
• ... but it doesn't work for dicts (#10).
  • In that case we may want to iterate on keys of the dict, but it would introduce a small discrepancy between arrays and dicts.
  • We can also forbid to iterate directly on dicts, and introduce functions such as keys(x) or values(x) that returns an array of the keys/values of x. They would also work to get the keys of an array.
  • Or we can say that a foreach loop always iterate on the keys, even for arrays:

    #PARAMETERS
       x = import "long_file_with_unknown_length.csv";
    //...
    #CONSTRAINTS
       a == sum(x[i] for i in x)

I believe the most "natural" way of handling things is either "value on arrays/keys on dicts" or "always value on arrays, forbid dict iteration, and keys(x)/values(x) to work on dicts".

Alternatives This can be done, for arrays only, by creating a function that returns the size of an array (#15).