Implement ast_case::compile for compiler series and reference code

code/compiler/06/ast.cpp

@@ -176,7 +176,51 @@ void ast_case::resolve(const type_mgr& mgr) const {
 }
 
 void ast_case::compile(const env_ptr& env, std::vector<instruction_ptr>& into) const {
+    type_data* type = dynamic_cast<type_data*>(node_type.get());
 
+    of->compile(env, into);
+    into.push_back(instruction_ptr(new instruction_eval()));
+
+    instruction_jump* jump_instruction = new instruction_jump();
+    into.push_back(instruction_ptr(jump_instruction));
+    for(auto& branch : branches) {
+        std::vector<instruction_ptr> branch_instructions;
+        pattern_var* vpat;
+        pattern_constr* cpat;
+
+        if((vpat = dynamic_cast<pattern_var*>(branch->pat.get()))) {
+            branch->expr->compile(env_ptr(new env_offset(1, env)), branch_instructions);
+
+            for(auto& constr_pair : type->constructors) {
+                if(jump_instruction->tag_mappings.find(constr_pair.second.tag) !=
+                        jump_instruction->tag_mappings.end())
+                    break;
+
+                jump_instruction->tag_mappings[constr_pair.second.tag] =
+                    jump_instruction->branches.size();
+            }
+            jump_instruction->branches.push_back(std::move(branch_instructions));
+        } else if((cpat = dynamic_cast<pattern_constr*>(branch->pat.get()))) {
+            branch_instructions.push_back(instruction_ptr(new instruction_split()));
+            branch->expr->compile(env_ptr(new env_offset(cpat->params.size(), env)),
+                branch_instructions);
+
+            int new_tag = type->constructors[cpat->constr].tag;
+            if(jump_instruction->tag_mappings.find(new_tag) !=
+                    jump_instruction->tag_mappings.end())
+                throw type_error("technically not a type error: duplicate pattern");
+
+            jump_instruction->tag_mappings[new_tag] =
+                jump_instruction->branches.size();
+            jump_instruction->branches.push_back(std::move(branch_instructions));
+        }
+    }
+
+    for(auto& constr_pair : type->constructors) {
+        if(jump_instruction->tag_mappings.find(constr_pair.second.tag) ==
+                jump_instruction->tag_mappings.end())
+            throw type_error("non-total pattern");
+    }
 }
 
 void pattern_var::print(std::ostream& to) const {

code/compiler/06/instruction.hpp

@@ -2,6 +2,8 @@
 #include <string>
 #include <memory>
 #include "binop.hpp"
+#include <vector>
+#include <map>
 
 struct instruction {
     virtual ~instruction() = default;
@@ -53,6 +55,11 @@ struct instruction_split : public instruction {
     
 };
 
+struct instruction_jump : public instruction {
+    std::vector<std::vector<instruction_ptr>> branches;
+    std::map<int, int> tag_mappings;
+};
+
 struct instruction_slide : public instruction {
     int offset;
 

content/blog/06_compiler_semantics.md

@@ -241,16 +241,16 @@ And now, we begin our implementation. Let's start with the easy ones:
 `ast_int`, `ast_lid` and `ast_uid`. The code for `ast_int` involves just pushing
 the integer into the stack:
 
-{{< codelines "C++" "compiler/06/ast.cpp" 18 20 >}}
+{{< codelines "C++" "compiler/06/ast.cpp" 36 38 >}}
 
 The code for `ast_lid` needs to check if the variable is global or local,
 just like we discussed:
 
-{{< codelines "C++" "compiler/06/ast.cpp" 31 36 >}}
+{{< codelines "C++" "compiler/06/ast.cpp" 53 58 >}}
 
 We do not have to do this for `ast_uid`:
 
-{{< codelines "C++" "compiler/06/ast.cpp" 47 49 >}}
+{{< codelines "C++" "compiler/06/ast.cpp" 73 75 >}}
 
 On to `ast_binop`! This is the first time we have to change our environment.
 As we said earlier, once we build the right operand on the stack, every offset that we counted
@@ -259,14 +259,14 @@ in our compilation scheme for function application). So,
 we create a new environment with `env_offset`, and use that
 when we compile the left child:
 
-{{< codelines "C++" "compiler/06/ast.cpp" 72 79 >}}
+{{< codelines "C++" "compiler/06/ast.cpp" 103 110 >}}
 
 `ast_binop` performs two applications: `(+) lhs rhs`. 
 We push `rhs`, then `lhs`, then `(+)`, and then use MkApp
 twice. In `ast_app`, we only need to perform one application,
 `lhs rhs`:
 
-{{< codelines "C++" "compiler/06/ast.cpp" 98 102 >}}
+{{< codelines "C++" "compiler/06/ast.cpp" 134 138 >}}
 
 Note that we also extend our environment in this one,
 for the exact same reason as before.
@@ -278,14 +278,15 @@ We need to adjust our code to keep track of the tags of the various
 constructors of a type. To do this, we add a subclass for the `type_base`
 struct, called `type_data`:
 
-{{< todo >}}Link code{{< /todo >}}
+{{< codelines "C++" "compiler/06/type.hpp" 33 42 >}}
 
 When we create types from `definition_data`, we tag the corresponding constructors:
 
-{{< todo >}}Link code{{< /todo >}}
+{{< codelines "C++" "compiler/06/definition.cpp" 35 51 >}}
 
-Ah, but that doesn't solve the problem. Once we performed type checking, we don't keep
-the types that we computed for an AST node in the node. And obviously, we don't want
+Ah, but adding constructor info to the type doesn't solve the problem.
+Once we performed type checking, we don't keep
+the types that we computed for an AST node, in the node. And obviously, we don't want
 to go looking for them again. Furthermore, we can't just look up a constructor
 in the environment, since we can well have patterns that don't have __any__ constructors:
 
@@ -296,11 +297,8 @@ match l {
 ```
 
 So, we want each `ast` node to store its type (well, in practice we only need this for
-`ast_case`, but we might as well store it for all nodes). We can add it, no problem:
-
-{{< todo >}}Link code{{< /todo >}}
-
-Now, we can add another, non-virtual `typecheck` method (let's call it `typecheck_common`,
+`ast_case`, but we might as well store it for all nodes). We can add it, no problem.
+To add to that, we can add another, non-virtual `typecheck` method (let's call it `typecheck_common`,
 since naming is hard). This method will call `typecheck`, and store the output into
 the `node_type` field.
 
@@ -311,7 +309,7 @@ type_ptr typecheck_common(type_mgr& mgr, const type_env& env);
 
 And the implementation is as simple as you think:
 
-{{< todo >}}Link code{{< /todo >}}
+{{< codelines "C++" "compiler/06/ast.cpp" 9 12 >}}
 
 In client code (`definition_defn::typecheck_first` for instance), we should now
 use `typecheck_common` instead of `typecheck`. With that done, we're almost there.
@@ -329,26 +327,76 @@ virtual void resolve(const type_mgr& mgr) const = 0;
 ```
 
 We also add the `resolve` method to `definition`, so that we can call it
-without having to run `dynamic_cast`. The implementation for `resolve_common`
+without having to run `dynamic_cast`. The implementation for `ast::resolve_common`
 just resolves the type:
 
-{{< todo >}}Link code{{< /todo >}}
+{{< codelines "C++" "compiler/06/ast.cpp" 14 21 >}}
 
-The virtual `resolve` just calls `resolve_common` on an all `ast` children
+The virtual `ast::resolve` just calls `ast::resolve_common` on an all `ast` children
 of a node. Here's a sample implementation from `ast_binop`:
 
-{{< todo >}}Link code{{< /todo >}}
+{{< codelines "C++" "compiler/06/ast.cpp" 98 101 >}}
 
-And here's the implementation of `resolve` on `definition_defn`:
+And here's the implementation of `definition::resolve` on `definition_defn`:
 
-{{< todo >}}Link code{{< /todo >}}
+{{< codelines "C++" "compiler/06/definition.cpp" 31 33 >}}
 
-Finally, we call `resolve` from inside `typecheck_program` in `main.cpp`:
+Finally, we call `resolve` at the end `typecheck_program` in `main.cpp`:
 
-{{< todo >}}Link code{{< /todo >}}
+{{< codelines "C++" "compiler/06/main.cpp" 40 42 >}}
 
-Finally, we're ready to implement the code for compiling `ast_case`.
+At last, we're ready to implement the code for compiling `ast_case`.
+Here it is, in all its glory:
 
-{{< todo >}}Figure out how to keep all trees not requiring a type manager. {{< /todo >}}
+{{< codelines "C++" "compiler/06/ast.cpp" 178 224 >}}
+
+There's a lot to unpack here. First of all, just like we said in the compilation
+scheme, we want to build and evaluate the expression that's being analyzed.
+Once that's done, however, things get more tricky. We know that each
+branch of a case expression will correspond to a vector of instructions -
+in fact, our jump instruction contains a mapping from tags to instructions.
+As we also discussed above, each list of instructions can be mapped to
+by multiple tags. We don't want to recompile the same sequence of instructions
+multiple times (or indeed, generate machine code for it). So, we keep
+a mapping of tags to their corresponding sequences of instructions. We implement
+this by having a vector of vectors of instructions (in which each inner vector
+represents the code for a branch), and a map of tag number to index
+in the vector containing all the branches. This way, multiple tags
+can point to the same instruction set without duplicating information.
+
+We also don't allow a tag to be mapped to more than one sequence of instructions.
+This is handled differently depending on whether a variable pattern or a
+constructor pattern are encountered. Variable patterns map all
+tags that haven't been mapped yet, so no error can occur. Constructor patterns,
+though, can explicitly try to map the same tag twice, and we don't want that.
+
+I implied in the previous paragraph the implementation of our case expression
+compilation algorithm, but let's go through it. Once we've compiled
+the expression to be analyzed, and evaluated it (just like in our definitions
+above), we proceed to look at all the branches specified in the case expression.
+
+If a branch has a variable pattern, we must map to the result of the compilation
+all the remaining, unmapped tags. We also aren't going to be taking apart
+our value, so we don't need to use Split, but we do need to add 1 to the
+environment offset to account the the presence of that value. So,
+we compile the branch body with that offset, and iterate through
+all the constructors of our data type. We skip a constructor
+if it's been mapped, and if it hasn't been, we map it to the index
+that this branch body will have in our list. Finally,
+we push the newly compiled instruction sequence into the list of branch
+bodies.
+
+If a branch is a constructor pattern, on the other hand, we lead our compilation
+output with a Split. This takes off the value from the stack, but pushes on
+all the parameters of the constructor. We account for this by incrementing the
+environment with the offset given by the number of arguments (just like we did
+in our definitions of our compilation scheme). Before we map the tag,
+we ensure that it hasn't already been mapped (and throw an exception, currently
+in the form of a type error due to the growing length of this post),
+and finally map it and insert the new branch code into the list of branches.
+
+After we're done with all the branches, we also check for non-exhaustive patterns,
+since otherwise we could run into runtime errors. With this, the case expression,
+an the last of the AST nodes, can be compiled.
 
 {{< todo >}}Backport bugfix in case's typecheck{{< /todo >}}