User Interface Issues

[jerryfung110]

1. Lack of Expression Support:

ExaModels does not support expressions like JuMP, requiring users to input long and complicated expressions simultaneously, making it unfriendly.

2. Iteration Limitation:

ExaModels cannot iterate a variable based on another variable within objective functions or constraints, expression cannot be represented concisely.

3. Array Indexing Issue:

Arrays not generated by ExaModels.variable() cannot be indexed directly in objective functions and constraints. Attempting to do so results in errors like an invalid index: ExaModels.ParIndexed, because ExaModels creates an ExaModels.ParIndexed object instead of an Int, causing indexing issues.

4. Unclear Indexing in Constraints:

The indexing syntax in constraint! is unclear, which can lead to confusion and errors in defining constraints when users try to add expressions in constraint!

[jbcaillau]

@sshin23 AFAIR regarding arrays (declared as variable), it is not possible to use slices (only element wise operations combined with generators)

[sshin23]

@jbcaillau do you mean something like this?

x = variable(c, 1:10)
y = view(x, 1:4)

It is currently not supported

[jbcaillau]

@sshin23 yes. more generally being able to do some basic linear algebra (BLAS1-2) operations. eg,

A = rand(10, 10)
x = variable(c, 1:10)
A[1, 1] * x[1] # OK
A[1, :]' * x # not defined
A * x # not defined
x[1:3] # not defined
... 

[jbcaillau]

hi @sshin23 @michel2323 quick follow up to our discussion last week (thanks for that). i've added a naive extension of Array to AbstractNode to allow basic operations on symbolic arrays coming from ExaModels. it is convenient for modelling purposes in OptimalControl.jl. the code is here, currently only used internally for tests:

https://github.com/control-toolbox/CTParser.jl/blob/vectorisation-three/test/exa_linalg.jl

this allows to use linear algebra in small dimensions without overhead in the size of the eventually generated expressions. to be rediscussed.

cc: @amontoison @jerryfung110