More ergonomic initialization for shifted variables

Author: YingboMa

src/discretedomain.jl

@@ -27,6 +27,7 @@ struct Shift <: Operator
     steps::Int
     Shift(t, steps = 1) = new(value(t), steps)
 end
+normalize_to_differential(s::Shift) = Differential(s.t)^s.steps
 function (D::Shift)(x, allow_zero = false)
     !allow_zero && D.steps == 0 && return x
     Term{symtype(x)}(D, Any[x])

src/utils.jl

@@ -858,3 +858,5 @@ function fast_substitute(expr, pair::Pair)
                 symtype(expr);
                 metadata = metadata(expr))
 end
+
+normalize_to_differential(s) = s

src/variables.jl

@@ -36,6 +36,17 @@ isoutput(x) = isvarkind(VariableOutput, x)
 isirreducible(x) = isvarkind(VariableIrreducible, x)
 state_priority(x) = convert(Float64, getmetadata(x, VariableStatePriority, 0.0))::Float64
 
+function default_toterm(x)
+    if istree(x) && (op = operation(x)) isa Operator
+        if !(op isa Differential)
+            x = normalize_to_differential(op)(arguments(x)...)
+        end
+        Symbolics.diff2term(x)
+    else
+        x
+    end
+end
+
 """
 $(SIGNATURES)
 
@@ -44,7 +55,7 @@ and creates the array of values in the correct order with default values when
 applicable.
 """
 function varmap_to_vars(varmap, varlist; defaults = Dict(), check = true,
-                        toterm = Symbolics.diff2term, promotetoconcrete = nothing,
+                        toterm = default_toterm, promotetoconcrete = nothing,
                         tofloat = true, use_union = false)
     varlist = collect(map(unwrap, varlist))
 

test/clock.jl

@@ -113,7 +113,8 @@ eqs = [yd ~ Sample(t, dt)(y)
        ]
 @named sys = ODESystem(eqs)
 ss = structural_simplify(sys)
-prob = ODEProblem(ss, [x => 0.0, y => 0.0], (0.0, 1.0), [kp => 1.0; z => 0.0; D(z) => 0.0])
+prob = ODEProblem(ss, [x => 0.0, y => 0.0], (0.0, 1.0),
+                  [kp => 1.0; z => 0.0; z(k + 1) => 0.0])
 sol = solve(prob, Tsit5(), kwargshandle = KeywordArgSilent)
 # For all inputs in parameters, just initialize them to 0.0, and then set them
 # in the callback.