feat: allow conditional var creation

src/model.jl

@@ -279,17 +279,12 @@ function generate_model_expression(backend, model_options, model_specification)
         end
         return expression
     end
-
-    varids = Set{Symbol}(ms_args_ids)
 
     # Step 2: Main pass
     ms_body = postwalk(ms_body) do expression
         # Step 2.1 Convert datavar calls
         if @capture(expression, varexpr_ = datavar(arguments__; options__)) 
-            @assert varexpr ∉ varids "Invalid model specification: '$varexpr' id is duplicated"
             @assert length(arguments) >= 1 "The expression `$expression` is incorrect. datavar(::Type, [ dims... ]) requires `Type` as a first argument."
-
-            push!(varids, varexpr)
 
             type_argument  = arguments[1]
             tail_arguments = arguments[2:end]
@@ -298,34 +293,29 @@ function generate_model_expression(backend, model_options, model_specification)
             return write_datavar_expression(backend, model, varexpr, dvoptions, type_argument, tail_arguments)
         # Step 2.2 Convert randomvar calls
         elseif @capture(expression, varexpr_ = randomvar(arguments__; options__))
-            @assert varexpr ∉ varids "Invalid model specification: '$varexpr' id is duplicated"
-            push!(varids, varexpr)
-
             rvoptions = write_randomvar_options(backend, varexpr, options)
-
             return write_randomvar_expression(backend, model, varexpr, rvoptions, arguments)
         # Step 2.3 Conver constvar calls 
         elseif @capture(expression, varexpr_ = constvar(arguments__))
-            @assert varexpr ∉ varids "Invalid model specification: '$varexpr' id is duplicated"
-            push!(varids, varexpr)
-
             return write_constvar_expression(backend, model, varexpr, arguments)
         # Step 2.2 Convert tilde expressions
         elseif @capture(expression, (nodeexpr_, varexpr_) ~ fform_(arguments__; kwarguments__))
-            # println(expression)
+
             varexpr, short_id, full_id = parse_varexpr(varexpr)
 
             if short_id ∈ bannedids
-                error("Invalid name '$(short_id)' for new random variable. '$(short_id)' was already initialized with '=' operator before.")
+                error("Invalid name '$(short_id)' for new random variable. '$(short_id)' has been already initialized with '=' operator.")
             end
 
             variables = map((argexpr) -> write_as_variable(backend, model, argexpr), arguments)
             options = write_node_options(backend, model, fform, [ varexpr, arguments... ], kwarguments)
-
-            if short_id ∈ varids
+
+            # Indexed variables like `y[1]` cannot be created on the fly and should be pre-initialised with `y = randomvar(n)`
+            # Single variables like `y` can be created on the fly with the `AutoVar` marker
+            # In the second case if variable `y` has been initialised before `AutoVar` should simply return it
+            if isref(varexpr)
                 return write_make_node_expression(backend, model, fform, variables, options, nodeexpr, varexpr)
             else
-                push!(varids, short_id)
                 return write_autovar_make_node_expression(backend, model, fform, variables, options, nodeexpr, varexpr, full_id)
             end
         else

src/utils.jl

@@ -41,6 +41,15 @@ See also: [`ishead`](@ref)
 """
 isref(expr) = ishead(expr, :ref)
 
+"""
+    getref(expr)
+
+Returns ref indices from `expr` in a form of a tuple.
+
+See als: [`isref`](@ref)
+"""
+getref(expr) = isref(expr) ? (view(expr.args, 2:lastindex(expr.args))...,) : ()
+
 """
     ensure_type(x)