More fully document the IR.

src/ir/ast.rs

@@ -13,6 +13,16 @@ use proptest::{
 type Variable = ArcIntern<String>;
 
 /// The representation of a program within our IR. For now, this is exactly one file.
+/// 
+/// In addition, for the moment there's not really much of interest to hold here besides
+/// the list of statements read from the file. Order is important. In the future, you
+/// could imagine caching analysis information in this structure.
+/// 
+/// `Program` implements both [`Pretty`] and [`Arbitrary`]. The former should be used
+/// to print the structure whenever possible, especially if you value your or your
+/// user's time. The latter is useful for testing that conversions of `Program` retain
+/// their meaning. All `Program`s generated through [`Arbitrary`] are guaranteed to be
+/// syntactically valid, although they may contain runtime issue like over- or underflow.
 #[derive(Debug)]
 pub struct Program {
     // For now, a program is just a vector of statements. In the future, we'll probably
@@ -56,6 +66,12 @@ impl Arbitrary for Program {
 /// 
 /// For now, this is either a binding site (`x = 4`) or a print statement
 /// (`print x`). Someday, though, more!
+/// 
+/// As with `Program`, this type implements [`Pretty`], which should
+/// be used to display the structure whenever possible. It does not
+/// implement [`Arbitrary`], though, mostly because it's slightly
+/// complicated to do so.
+/// 
 #[derive(Debug)]
 pub enum Statement {
     Binding(Location, Variable, Expression),
@@ -84,6 +100,17 @@ where
 }
 
 /// The representation of an expression.
+/// 
+/// Note that expressions, like everything else in this syntax tree,
+/// supports [`Pretty`], and it's strongly encouraged that you use
+/// that trait/module when printing these structures.
+/// 
+/// Also, Expressions at this point in the compiler are explicitly
+/// defined so that they are *not* recursive. By this point, if an
+/// expression requires some other data (like, for example, invoking
+/// a primitive), any subexpressions have been bound to variables so
+/// that the referenced data will always either be a constant or a
+/// variable reference.
 #[derive(Debug)]
 pub enum Expression {
     Value(Location, Value),
@@ -120,6 +147,12 @@ where
     }
 }
 
+/// A type representing the primitives allowed in the language.
+/// 
+/// Having this as an enumeration avoids a lot of "this should not happen"
+/// cases, but might prove to be cumbersome in the future. If that happens,
+/// this may either become a more hierarchical enumeration, or we'll just
+/// deal with the "this should not happen" cases.
 #[derive(Clone, Copy, Debug, Eq, PartialEq)]
 pub enum Primitive {
     Plus,
@@ -157,6 +190,11 @@ where
     }
 }
 
+/// An expression that is always either a value or a reference.
+/// 
+/// This is the type used to guarantee that we don't nest expressions
+/// at this level. Instead, expressions that take arguments take one
+/// of these, which can only be a constant or a reference.
 #[derive(Debug)]
 pub enum ValueOrRef {
     Value(Location, Value),
@@ -185,8 +223,14 @@ impl From<ValueOrRef> for Expression {
     }
 }
 
+/// A constant in the IR.
 #[derive(Debug)]
 pub enum Value {
+    /// A numerical constant.
+    /// 
+    /// The optional argument is the base that was used by the user to input
+    /// the number. By retaining it, we can ensure that if we need to print the
+    /// number back out, we can do so in the form that the user entered it.
     Number(Option<u8>, i64),
 }