docs/functional: Add dummy toy example

Add `dummy.cc`, loosely based on `backends/functional/test_generic.cc` but as an actualy backend and without the memory testing.
Skeleton section for minimal functional backend, describing the aforementioned `dummy.cc`.

docs/source/code_examples/functional/dummy.cc

@@ -0,0 +1,44 @@
+#include "kernel/functional.h"
+#include "kernel/yosys.h"
+
+USING_YOSYS_NAMESPACE
+PRIVATE_NAMESPACE_BEGIN
+
+struct FunctionalDummyBackend : public Backend {
+	FunctionalDummyBackend() : Backend("functional_dummy", "dump generated Functional IR") {}
+	void execute(std::ostream *&f, std::string filename, std::vector<std::string> args, RTLIL::Design *design) override
+	{
+		// backend pass boiler plate
+		log_header(design, "Executing dummy functional backend.\n");
+
+		size_t argidx = 1;
+		extra_args(f, filename, args, argidx, design);
+
+		for (auto module : design->selected_modules())
+		{
+			log("Processing module `%s`.\n", module->name.c_str());
+
+			// convert module to FunctionalIR
+			auto ir = Functional::IR::from_module(module);
+			*f << "module " << module->name.c_str() << "\n";
+
+			// write node functions
+			for (auto node : ir)
+				*f << "  assign " << id2cstr(node.name())
+				   << " = " << node.to_string() << "\n";
+			*f << "\n";
+
+			// write outputs and next state
+			for (auto output : ir.outputs())
+				*f << " " << id2cstr(output->kind)
+				   << " " << id2cstr(output->name)
+				   << " = " << id2cstr(output->value().name()) << "\n";
+			for (auto state : ir.states())
+				*f << " " << id2cstr(state->kind)
+				   << " " << id2cstr(state->name)
+				   << " = " << id2cstr(state->next_value().name()) << "\n";
+		}
+	}
+} FunctionalDummyBackend;
+
+PRIVATE_NAMESPACE_END

docs/source/yosys_internals/extending_yosys/functional_ir.rst

@@ -144,6 +144,41 @@ S-expressions can be constructed with ``SExpr::list``, for example ``SExpr expr
   parentheses.
 - The destructor calls ``flush`` but also closes all unclosed parentheses.
 
+Example: A minimal functional backend
+-------------------------------------
+
+.. literalinclude:: /code_examples/functional/dummy.cc
+   :language: c++
+   :caption: Example source code for a minimal functional backend
+
+- three main steps needed
+
+  + convert to FunctionalIR
+  + handle nodes
+  + handle outputs and next state
+
+- backend pass boiler plate gives us ``write_functional_dummy`` command
+- pointer ``f`` is a ``std::ostream`` we can write to, being either a file or
+  stdout
+- FunctionalIR conversion done by ``Functional::IR::from_module()``
+- each node performs some function or operation (including reading input/current
+  state)
+
+  + each variable is assigned exactly once before being used
+  + ``node.name()`` returns a ``RTLIL::IdString``, which we convert for
+    printing with ``id2cstr()``
+  + ``node.to_string()`` converts the node into a string of the form
+    ``function(args)``
+
+    * ``function`` is the result of ``Functional::IR::fn_to_string(node.fn())``
+    * ``args`` are the zero or more arguments passed to the function, most
+      commonly the name of another node
+    * wraps ``node.visit()`` with a private visitor with return type
+      ``std::string``
+
+- ``Functional::IROutput::value()`` and ``Functional::IRState::next_value()``
+  represent the outputs of our function
+
 Example: Adapting SMT-LIB backend for Rosette
 ---------------------------------------------