---
title: Compiling a Functional Language Using C++, Part 4 - Small Improvements
date: 2019-08-06T14:26:38-07:00
draft: true
tags: ["C and C++", "Functional Languages", "Compilers"]
---
We've done quite a big push in the previous post. We defined
type rules for our language, implemented unification,
and then implemented unification to enforce these rules for
our program. The post was pretty long, and even then we
weren't able to fit quite everything into it.

For instance, we threw 0 whenever an error occured. This
gives us no indication of what actually went wrong. We should
probably define an exception class, one that can contain
information about the error, and report it to the user.

Also, when there's no error, our compiler doesn't
really tell us anything at all about the code besides
the number of definitions. We probably want to see the types
of these definitions, or at least some intermediate information.
At the very least, we want to have the __ability__ to see
this information.

Finally, we have no build system. We are creating more
and more source files, and so far (unless you've taken
initiative), we've been compiling them by hand. We want
to only compile source files that have changed,
and we want to have a standard definition of how to
build our program.

### Printing Syntax Trees
Let's start by printing the trees we get from our parser.
This is long overdue - we had no way to verify the structure
of what our parser returned to us since Part 2. We'll print
the trees top-down, with the children of a node
indent one block further than the node itself. For this,
we'll make a new virtual function with the signature:
```
virtual void print(int indent, std::ostream& to) const;
```
We'll include a similar printing function into our
pattern struct, too:
```
virtual void print(std::ostream& to) const;
```

Let's take a look at the implementation. For `ast_int`,
`ast_lid`, and `ast_uid`:
{{< codelines "C++" "compiler/04/ast.cpp" 18 21 >}}
{{< codelines "C++" "compiler/04/ast.cpp" 27 30 >}}
{{< codelines "C++" "compiler/04/ast.cpp" 36 39 >}}

With `ast_binop` things get a bit more interesting.
We call `print` recursively on the children of the
`binop` node:
{{< codelines "C++" "compiler/04/ast.cpp" 45 50 >}}

The same idea for `ast_app`:
{{< codelines "C++" "compiler/04/ast.cpp" 66 71 >}}

Finally, just like `ast_case::typecheck` called 
`pattern::match`, `ast_case::print` calls `pattern::print`:
{{< codelines "C++" "compiler/04/ast.cpp" 83 92 >}}

We follow the same implementation strategy for patterns,
but we don't need indentation, or recursion:
{{< codelines "C++" "compiler/04/ast.cpp" 108 110 >}}
{{< codelines "C++" "compiler/04/ast.cpp" 116 121 >}}

Let's print the bodies of each function we receive from the parser:
{{< codelines "C++" "compiler/04/main.cpp" 35 50 >}}

### Printing Types
Types are another thing that we want to be able to inspect, so let's
add a similar print method to them:
```
virtual void print(const type_mgr& mgr, std::ostream& to) const;
```
We need the type manager so we can follow substitutions.
The implementation is simple enough:
{{< codelines "C++" "compiler/04/type.cpp" 5 24 >}}

Let's also print out the types we infer. We'll make it a separate loop
in the `typecheck_program` function, because it's mostly just
for debugging purposes.

### Fixing Bugs
We actually discover not one, but two bugs in our implementation thanks
to this output. Observe the output for `works3.txt`:
```
length l:
  CASE:
    Nil
      INT: 0
*: Int -> (Int -> (Int))
+: Int -> (Int -> (Int))
-: Int -> (Int -> (Int))
/: Int -> (Int -> (Int))
Cons: List -> (Int -> (List))
Nil: List
length: List -> (Int)
2
```

First, we're missing the `Cons` branch. The culprit is `parser.y`, specifically
this line:
```C++
    : branches branch { $$ = std::move($1); $1.push_back(std::move($2)); }
```
Notice that we move our list of branches out of `$1`. However, when we
`push_back`, we use `$1` again. That's wrong! We need to `push_back`
to `$$` instead:
{{< codelines "C++" "compiler/04/parser.y" 110 110 >}}

Next, observe that `Cons` has type `List -> Int -> List`. That's not right,
since `Int` comes first in our definition. The culprit is this fragment of code:
```C++
        for(auto& type_name : constructor->types) {
            type_ptr type = type_ptr(new type_base(type_name));
            full_type = type_ptr(new type_arr(type, full_type));
        }
```
Remember how we build the function type backwards in Part 3? We have to do the same here.
We replace the fragment with the proper reverse iteration:
{{< codelines "C++" "compiler/04/definition.cpp" 37 40 >}}


### Setting up CMake
This would be extremely easy if not for Flex and Bison. We start with the usual:
{{< codelines "CMake" "compiler/04/CMakeLists.txt" 1 2 >}}

Next, we want to set up Flex and Bison. CMake provides two commands for this:
{{< codelines "CMake" "compiler/04/CMakeLists.txt" 4 5 >}}

We now have access to commands that allow us to tell CMake about our parser
and tokenizer (or scanner). We use them as follows:
{{< codelines "CMake" "compiler/04/CMakeLists.txt" 6 12 >}}

We also want CMake to know that the scanner needs to parser's header file
in order to compile. We add this dependency:
{{< codelines "CMake" "compiler/04/CMakeLists.txt" 13 13 >}}

Finally, we add our source code to a CMake target. We use
the `BISON_parser_OUTPUTS` and `FLEX_scanner_OUTPUTS` to
pass in the source files generated by Flex and Bison.
{{< codelines "CMake" "compiler/04/CMakeLists.txt" 15 22 >}}

Almost there! `parser.cpp` will be generated in the `build` directory
during an out-of-source build, and so will `parser.hpp`. When building,
`parser.cpp` will try to look for `ast.hpp`, and `main.cpp` will look for
`parser.hpp`. We want them to be able to find each other, so we
add both the source directory and the build (binary) directory to
the list of includes directories:

{{< codelines "CMake" "compiler/04/CMakeLists.txt" 23 24 >}}

That's it for CMake! Let's try our build:
```
cmake -S . -B build
cd build && make -j8
```

We get an executable called `compiler`. Excellent! Here's the whole file:
{{< codeblock "CMake" "compiler/04/CMakeLists.txt" >}}