feat: support Differential(t) syntax in parse_variable

Author: AayushSabharwal

src/systems/abstractsystem.jl

@@ -3292,7 +3292,7 @@ Return the variable in `sys` referred to by its string representation `str`.
 Roughly supports the following CFG:
 
 ```
-varname                  = "D(" varname ")" | arrvar | maybe_dummy_var
+varname                  = "D(" varname ")" | "Differential(" iv ")(" varname ")" | arrvar | maybe_dummy_var
 arrvar                   = maybe_dummy_var "[idxs...]"
 idxs                     = int | int "," idxs
 maybe_dummy_var          = namespacedvar | namespacedvar "(" iv ")" |
@@ -3310,9 +3310,18 @@ function parse_variable(sys::AbstractSystem, str::AbstractString)
     # I'd write a regex to validate `str`, but https://xkcd.com/1171/
     str = strip(str)
     derivative_level = 0
-    while startswith(str, "D(") && endswith(str, ")")
+    while ((cond1 = startswith(str, "D(")) || startswith(str, "Differential(")) && endswith(str, ")")
+        if cond1
+            derivative_level += 1
+            str = _string_view_inner(str, 2, 1)
+            continue
+        end
+        _tmpstr = _string_view_inner(str, 13, 1)
+        if !startswith(_tmpstr, "$iv)(")
+            throw(ArgumentError("Expected differential with respect to independent variable $iv in $str"))
+        end
         derivative_level += 1
-        str = _string_view_inner(str, 2, 1)
+        str = _string_view_inner(_tmpstr, length(iv) + 2, 0)
     end
 
     arr_idxs = nothing

test/variable_utils.jl

@@ -88,6 +88,10 @@ end
         ("D(😄.x($iv))", D(sys.😄.x)),
         ("D(😄₊x)", D(sys.😄.x)),
         ("D(😄₊x($iv))", D(sys.😄.x)),
+        ("Differential($iv)(😄.x)", D(sys.😄.x)),
+        ("Differential($iv)(😄.x($iv))", D(sys.😄.x)),
+        ("Differential($iv)(😄₊x)", D(sys.😄.x)),
+        ("Differential($iv)(😄₊x($iv))", D(sys.😄.x)),
         # other derivative
         ("😄.xˍ$iv", D(sys.😄.x)),
         ("😄.x($iv)ˍ$iv", D(sys.😄.x)),
@@ -113,6 +117,12 @@ end
         ("D(arr₊x[1])", D(sys.arr.x[1])),
         ("D(arr.x($iv)[1])", D(sys.arr.x[1])),
         ("D(arr₊x($iv)[1])", D(sys.arr.x[1])),
+        ("Differential($iv)(arr.x($iv))", D(sys.arr.x)),
+        ("Differential($iv)(arr₊x($iv))", D(sys.arr.x)),
+        ("Differential($iv)(arr.x[1])", D(sys.arr.x[1])),
+        ("Differential($iv)(arr₊x[1])", D(sys.arr.x[1])),
+        ("Differential($iv)(arr.x($iv)[1])", D(sys.arr.x[1])),
+        ("Differential($iv)(arr₊x($iv)[1])", D(sys.arr.x[1])),
         # other derivative
         ("arr.xˍ$iv", D(sys.arr.x)),
         ("arr₊xˍ$iv", D(sys.arr.x)),