feat: add label metadata to symbolic variables

docs/src/basics/Variable_metadata.md

@@ -39,6 +39,14 @@ help?> u
   Symbolics.VariableSource: (:variables, :u)
 ```
 
+## Label
+
+Label is descriptive name of a symbolic variable. It can be defined as:
+
+```@example metadata
+@parameters R [label = "Resistance"]
+```
+
 ## Connect
 
 Variables in connectors can have `connect` metadata which describes the type of connections.

src/ModelingToolkit.jl

@@ -204,7 +204,7 @@ export connect, domain_connect, @connector, Connection, Flow, Stream, instream
 export @component, @mtkmodel, @mtkbuild
 export isinput, isoutput, getbounds, hasbounds, isdisturbance, istunable, getdist, hasdist,
     tunable_parameters, isirreducible, getdescription, hasdescription, isbinaryvar,
-    isintegervar
+    isintegervar, getlabel, haslabel
 export ode_order_lowering, dae_order_lowering, liouville_transform
 export PDESystem
 export Differential, expand_derivatives, @derivatives

src/variables.jl

@@ -368,6 +368,25 @@ function hasdescription(x)
     getdescription(x) != ""
 end
 
+## Label =================================================================
+struct VariableLabel end
+Symbolics.option_to_metadata_type(::Val{:label}) = VariableLabel
+
+getlabel(x::Num) = getlabel(Symbolics.unwrap(x))
+
+"""
+    getlabel(x)
+
+Return labels attached to variables `x`. If no label is attached, an empty string is returned.
+"""
+function getlabel(x)
+    p = Symbolics.getparent(x, nothing)
+    p === nothing || (x = p)
+    Symbolics.getmetadata(x, VariableLabel, "")
+end
+
+haslabel(x) = (getlabel(x) != "")
+
 ## binary variables =================================================================
 struct VariableBinary end
 Symbolics.option_to_metadata_type(::Val{:binary}) = VariableBinary

test/test_variable_metadata.jl

@@ -76,6 +76,15 @@ sp = Set(p)
 @test getdescription(u) == ""
 @test !hasdescription(u)
 
+## Labels
+@variables l [label = "Label"]
+@test getlabel(l) == "Label"
+@test haslabel(l)
+
+@variables l
+@test getlabel(l) == ""
+@test !haslabel(l)
+
 @parameters t
 @variables u(t) [description = "A short description of u"]
 @parameters p [description = "A description of p"]