Start writing up the implementation

content/blog/06_compiler_semantics.md

@@ -161,9 +161,35 @@ graphs when we have to instantiate those functions, we simply
 evaluate the arguments and perform the relevant arithmetic operation using BinOp.
 We will do a similar thing for constructors.
 
-With that out of the way, we can get around to writing some code. We can envision
+### Implementation
-a method on the `ast` struct that takes an environment (just like our compilation
+
-scheme takes the environment \\(\\rho\\\)). Rather than returning a vector
+With that out of the way, we can get around to writing some code. Let's
+first define C++ structs for the instructions of the G-machine:
+
+{{< codeblock "C++" "compiler/06/instruction.hpp" >}}
+
+We can now envision a method on the `ast` struct that takes an environment 
+(just like our compilation scheme takes the environment \\(\\rho\\\)),
+and compiles the `ast`. Rather than returning a vector
 of instructions (which involves copying, unless we get some optimization kicking in),
-we'll pass to it a reference to a vector. The method will then place the generated
+we'll pass a reference to a vector to our method. The method will then place the generated
 instructions into the vector. 
+
+There's one more thing to be considered. How do we tell apart a "global"
+from a variable? A naive solution would be to take a list or map of
+global functions as a third parameter to our `compile` method.
+But there's an easier way! We know that the program passed type checking.
+This means that every referenced variable exists. From then, the situation is easy -
+if actual variable names are kept in the environment, \\(\\rho\\), then whenever
+we see a variable that __isn't__ in the current environment, it must be a function name.
+
+Having finished contemplating out method, it's time to define a signature:
+```C++
+virtual void compile(const env_ptr env, std::vector<instruction>& into) const;
+```
+
+Ah, but now we have to define "environment". Let's do that:
+
+{{< codeblock "C++" "compiler/06/env.hpp" >}}
+
+And now, we begin our implementation.