Add better error messages for getter function which are not supported

[ueliwechsler]

It's a small thing but consider an unconstrained system

sys = @system(x' = Ax + Bu)

and try to access a nonexisting field, the error message could be improved.

stateset(a)
ERROR: MethodError: no method matching stateset(::LinearControlDiscreteSystem{Float64,Array{Float64,2},Array{Float64,2}})
Closest candidates are:
  stateset(::ConstrainedContinuousIdentitySystem) at C:\Users\wueli\.julia\dev\MathematicalSystems\src\systems.jl:71
  stateset(::ConstrainedDiscreteIdentitySystem) at C:\Users\wueli\.julia\dev\MathematicalSystems\src\systems.jl:71
  stateset(::ConstrainedLinearContinuousSystem) at C:\Users\wueli\.julia\dev\MathematicalSystems\src\systems.jl:327
  ...
Stacktrace:
 [1] top-level scope at none:0

For example we could return:

ERROR: the state set cannot be accessed for a system of  type `LinearControlDiscreteSystem` 

[mforets]

yes, the current error message doesn't say too much and it can be improved, more explicitly ".. the state set and the input constraints must be specified for this type of system". On the other hand, i consider this only a temporary fix that we should be able to handle after #180, possibly filling such fields with nothing (again this can be changed later but it is a good step forward)

[ueliwechsler]

Another approach would be to return nothing for all types where stateset, noiseset etc are not defined. Which is preferable?

[mforets]

Which is preferable?

I don't know in general, but for example the approach in ReachabilityAnalysis is:

system  = @system(x' = Ax) # user defines a system
# ...  user associates this system with an initial-value problem and calls solve ...
# internally:
normalize(system) # returns a system with Universe as state constraints
discretize(..) ... reach(..) ...

So whenever one hits a system whose stateset is not defined, normalize (defined here) attaches a universal stateset. After normalization, algorithms can rely on having the stateset method to work and it's easier to write a generic algorithm.

[mforets]

for this issue, i think that improving the error message is fine. the code could be simplified using an interface (or traits) but there is no much done about that yet in this library.

[schillic]

On the other hand, i consider this only a temporary fix that we should be able to handle after #180, possibly filling such fields with nothing (again this can be changed later but it is a good step forward)

A LinearControlDiscreteSystem (as in the OP) will never have state constraints, even after #180. This issue is only about an undefined method.

Another approach would be to return nothing for all types where stateset, noiseset etc are not defined.

I prefer this behavior. Receiving nothing for something that does not exist is natural and easier to work with than an error message saying that you are asking for something that does not exist for a specific system instance. You can always override this behavior to return your favorite universe representation of course, but the default behavior with nothing sounds reasonable.