Add beginning of part 6 of compiler series

content/blog/06_compiler_semantics.md

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+---
+title: Compiling a Functional Language Using C++, Part 6 - Compilation
+date: 2019-08-06T14:26:38-07:00
+draft: true
+tags: ["C and C++", "Functional Languages", "Compilers"]
+---
+In the previous post, we defined a magine for graph reduction,
+called a G-machine. However, this machine is still not particularly
+connected to __our__ language. In this post, we will give
+meanings to programs in our language in the context of
+this G-machine. We will define a __compilation scheme__,
+which will be a set of rules that tell us how to
+translate programs in our language into G-machine instructions.
+To mirror _Implementing Functional Languages: a tutorial_, we'll
+call this compilation scheme \\(\\mathcal{C}\\), and write it
+as \\(\\mathcal{C} ⟦e⟧ = i\\), meaning "the expression \\(e\\)
+compiles to the instructions \\(i\\)".
+
+To follow our route from the typechecking, let's start
+with compiling expressions that are numbers. It's pretty easy:
+$$
+\\mathcal{C} ⟦n⟧ = [\\text{PushInt} \\; n]
+$$
+
+Here, we compiled a number expression to a list of
+instructions with only one element - PushInt.
+
+Just like when we did typechecking, let's
+move on to compiling function applications. As
+we informally stated in the previous chapter, since
+the thing we're applying has to be on top,
+we want to compile it last:
+
+$$
+\\mathcal{C} ⟦e\_1 \; e\_2⟧ = \\mathcal{C} ⟦e\_2⟧ ⧺ \\mathcal{C} ⟦e\_1⟧ ⧺ [\\text{MkApp}]
+$$
+
+Here, we used the \\(⧺\\) operator to represent the concatenation of two
+lists. Otherwise, this should be pretty intutive - we first run the instructions
+to create the parameter, then we run the instructions to create the function,
+and finally, we combine them using MkApp.
+
+It's variables that once again force us to adjust our strategy. If our
+program is well-typed, we know our variable will be on the stack:
+our definition of Unwind makes it so for functions, and we will
+define our case expression compilation scheme to match. However,
+we still need to know __where__ on the stack each variable is,
+and this changes as the stack is modified.
+
+To accommodate for this, we define an environment, \\(\\rho\\),
+to be a partial function mapping variable names to thier
+offsets on the stack. We write \\(\\rho = [x \\rightarrow n, y \\rightarrow m]\\)
+to say "the environment \\(\\rho\\) maps variable \\(x\\) to stack offset \\(n\\),
+and variable \\(y\\) to stack offset \\(m\\)". We also write \\(\\rho \; x\\) to
+say "look up \\(x\\) in \\(\\rho\\)", since \\(\\rho\\) is a function. Finally,
+to help with the ever-changing stack, we define an augmented environment
+\\(\\rho^{+n}\\), such that \\(\\rho^{+n} \; x = \\rho \; x + n\\). In words,
+this basically means "\\(\\rho^{+n}\\) has all the variables from \\(\\rho\\),
+but their addresses are incremented by \\(n\\)". We now pass \\(\\rho\\)
+in to \\(\\mathcal{C}\\) together with the expression \\(e\\). Let's
+rewrite our first two rules. For numbers:
+
+$$
+\\mathcal{C} ⟦n⟧ \; \\rho = [\\text{PushInt} \\; n]
+$$
+
+For function application:
+$$
+\\mathcal{C} ⟦e\_1 \; e\_2⟧ \; \\rho = \\mathcal{C} ⟦e\_2⟧ \; \\rho ⧺ \\mathcal{C} ⟦e\_1⟧ \; \\rho^{+1} ⧺ [\\text{MkApp}]
+$$
+
+Notice how in that last rule, we passed in \\(\\rho^{+1}\\) when compiling the function's expression. This is because
+the result of running the instructions for \\(e\_2\\) will have left on the stack the function's parameter. Whatever
+was at the top of the stack (and thus, had index 0), is now the second element from the top (address 1). The
+same is true for all other things that were on the stack. So, we increment the environment accordingly.
+
+With the environment, the variable rule is simple:
+$$
+\\mathcal{C} ⟦x⟧ \; \\rho = [\\text{Push} \\; (\\rho \; x)]
+$$
+
+One more thing. If we run across a function name, we want to
+use PushGlobal rather than Push. Defining \\(f\\) to be a name
+of a global function, we capture this using the following rule:
+
+$$
+\\mathcal{C} ⟦f⟧ \; \\rho = [\\text{PushGlobal} \\; f]
+$$
+
+Next up, case expressions. These are a bit more complex: there are several
+branches, each of which will have its own environment.