Proposal: change API for modifications

[odow]

To date, I'm not sure if anyone has dug into the modification API yet. I was about to start writing tests (#354) and had some thoughts.

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In addition to some function modifications, we currently have two modifyconstraint! methods:

modifyconstraint!(m, c::ConstraintIndex{F,S}, new_set::S)
modifyconstraint!(m, c::ConstraintIndex{F,S}, new_function::F)

Considering the rest of the API, doesn't it make more sense to make these set! methods?

set!(m::ModelLike, ::ConstraintFunction, c::ConstraintIndex{F,S}, new_function::F)
set!(m::ModelLike, ::ConstraintSet, c::ConstraintIndex{F,S}, new_set::S)

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Furthermore, instead of modifyconstraint! and modifyobjective!, we could have

canmodify(m, ::ConstraintFunction, c::Type{ConstraintIndex{F,S}}, change::Type{AbstractFunctionModification})
modify!(m, ::ConstraintFunction, c::ConstraintIndex{F,S}, change::AbstractFunctionModification)

canmodify(m, ::ObjectiveFunction, change::Type{AbstractFunctionModification})
modify!(m, ::ObjectiveFunction, change::AbstractFunctionModification)

Edit: we could also change the transformconstraint to

canmodify(m, ::ConstraintSet, c::Type{ConstraintIndex{F,S1}}, ::Type{S2})
c2 = modify!(m, ::ConstraintSet, c::ConstraintIndex{F,S1}, new_set::S2)

[blegat]

I am in favor of these changes.

For transform -> modify, I remember we named it transform to make it different than modify with a set of the same type but if modifying the set is moved to set! then we could add changing the set with a new set type to modify. However, transform need to return a new constraint index while modify does not change the constraint index so it is probably better to keep then separate functions.

[mlubin]

Agreed with @blegat. The change to set! and modify! looks good. I disagree with merging transformconstraint into modify! because the function has a different meaning that's inconsistent with what happens when the sets are of the same type.

[odow]

The majority of this was addressed by #388.

Closing in favor of #398.