# Minimum Viable Product

What features would make our compiler friendlier? My biggest gripes are that symbols must be defined before use, a finicky parser that lacks support for indentation rules, and pathetic error handling. We work on the first two problems, and make a tiny dent on the third, while taking care of a few other issues.

This leads to a compiler that I found surprisingly usable, at least when I could spot mistakes on my own.

## Mutually

C requires symbols to be declared before use. Our compilers are fussier still, as they require symbols to be completely defined before use. This irks programmers, especially when mutual recursion is desired, and also irks our compilers, because we must process functions and instance methods in the order they appear. This is particularly annoying when the two are interleaved.

Supporting arbitrary orderings of definitions requires changing multiple stages of our compiler.

We break type inference into 3 steps:

1. As we parse, we generate the type and abstract syntax tree of each data constructor and each typeclass method, adding them to pre-defined primitives.

2. We infer the types of top-level definitions. For this stage, we construct a dependency graph (that is, we determine the symbols required by each symbol) then find its strongly connected components. Each member of a component mutually depends on each other member, and we infer their types together. Our inferno function continually piles on more type constraints for each member of a component, and only resolves them after all have been processed.

3. We infer the type of instance method definitions, and check they are correct. A later compiler supports default class method definitions, which are also handled in this phase.

During code generation, we no longer know the address of a dependent symbol. Instead, we must leave space for an address and fill it in later. We take advantage of lazy tying-the-knot style so the code appears to effortlessly solve this problem.

As our previous now has a predefined Bool type, we take this opportunity to refactor to use if-then-else instead of matching on True and False.

▶ Toggle mutually.hs

## Uniquely

Now that code generation requires a map of addresses, it’s a good time to experiment with hash consing. We reduce heap usage my maximizing sharing. However, it may cost too much, as this iteration of our compiler is appreciably slower!

Consider definitions whose right-hand side is a lone variable. Our optiComb function follows lone variables so that:

f = g
g = h
h = f
x = (f, g, h)
y = x
z = y
w = (x, y, z)

compiles to:

f = g
h = g
g = Y I
x = (g, g, g)
y = x
z = x
w = (x, x, x)

That is, afterwards, a variable with a lone variable definition only appears on the right-hand side if its definition has been rewritten to fix id, so is no longer a lone variable. Our asm function relies on this, because it skips anything whose right-hand side is a lone variable.

This causes a corner case to fail: our compiler crashes on attempting to export a symbol whose right-hand side remains a lone variable after optiComb. For the time being, we let this slide.

We clean up top-level definitions as mutual recursion is now possible.

We add support for definitions appearing in any order in a let block. This is trickier than at the top-level, because of shared variable bindings floating around. Again, we find the strongly connected components to detect mutual dependencies, but instead of a table of addresses, we apply simple lambda lifting. See Peyton Jones and Lester, Implementing Functional Languages: a tutorial, Chapter 6.

In brief, we order the members of each component arbitrarily and insert variables so they can all reach each other; we automate what we did by hand when writing mutually recursive functions for older versions of our compiler. For example:

let
a = foo a b c
b = bar a b c
c = baz a b c
in qux a b c

is rewritten to the cycle-free:

let
a b c = foo (a b c) (b c) c
b c   = bar (a b c) (b c) c
c     = baz (a b c) (b c) c
in qux (a b c) (b c) c

A triangle appears on the left-hand side, explaining our choice of function name, and while the idea is straightforward, the implementation is tedious because we recurse in all sorts of ways over the non-empty tails of lists of variables, such as [[a, b, c], [b, c], [c]] and because we perform substitutions in the syntax tree while it still possibly contains case expressions and pattern matches.

The leftyPat function supports patterns on the left-hand side of definitions, for example:

[a,b,c] = expr

Our solution is simplistic. We find all pattern variables, such as a,b,c. If nonempty, we prepend @ to the first variable, for example @a, to generate a symbol unique to the current scope (a cheap trick to approximate Lisp’s gensym). Then we define this generated symbol to be the expression on the right-hand side, for example @a = expr, and then we generate case expressions for each pattern variable to define them, for example

@a = expr
a = case @a of [a,b,c] -> a
b = case @a of [a,b,c] -> b
c = case @a of [a,b,c] -> c

Our scheme fails to handle the wild-card pattern _ correctly, which we’ll fix in a later compiler. Until then, we tread carefully with patterns on the left.

▶ Toggle uniquely.hs

## Virtually

Concatenating the runtime system with the compiler output is tiresome. Our next compiler also generates the source for the virtual machine.

We change Int from unsigned to signed. We rename (/) and (%) to match Haskell’s div and mod, though they really should be quot and rem; we’ll fix this later.

We add support for newIORef, readIOref, and writeIORef. An IORef holding a value x of type a is represented as REF x where REF behaves like NUM:

REF x f --> f (REF x)

Thus an IORef takes one app-cell in our VM, which adds a layer of indirection. The address of this app-cell may be freely copied, and writeIORef can update all these copies at once, by changing a single entry. We hardwire the following:

newIORef value world cont = cont (REF value) world
writeIORef ref value world cont = ref (WRITEREF value) world cont
READREF (REF x) world cont = cont x world
WRITEREF value (REF _) world cont = cont () world

WRITEREF also has a side effect: it overwrites the given app-cell with REF value before returning cont. It is the only combinator that can modify the values in the heap, excluding changes caused by lazy updates and garbage collection.

▶ Toggle virtually.hs

## Marginally

Landin’s off-side rule is sorely missed. Although cosmetic, layout parsing rules give Haskell a clean mathematical look.

We split off a lexer from our parser, and follow the rules in section 10.3 of the Haskell 2010 spec.

We add support for multiple predicates in the context of an instance. We should have done this before, as it’s just a small parser tweak; the rest of the code can already handle it.

This is a good moment to support do notation. We deviate from the spec. Trailing let statements are legal; they just have no effect. It is also legal for the last statement to be a binding, in which case we implicitly follow it with pure ().

▶ Toggle marginally.hs

## Methodically

We correct a glaring defect. Up until now, the methods of an instance must be defined in the same order they are declared in their class, otherwise bad code is silently produced.

We add support for default methods as it involves the same code. Our simple approach insists the type of the default implementation of a method in a class Foo to have the constraint of the form Foo a =>, because we always pass a dictionary as the first argument. We could improve this slightly by inserting const in the syntax tree if we deduce no constraints are present.

We ruthlessly remove semicolons and braces from our source.

Now that the syntax is slightly more pleasant:

• We refine leftyPat so it correctly handles the wild-card pattern _ in the left-hand side of a definition.

• We support ranges, except for those that specify a step size.

• We support list comprehensions.

• To match GHC, we support foreign import ccall as well as ffi, and foreign export ccall as well as export. In the next compiler, we’ll remove ffi and plain export.

▶ Toggle methodically.hs

Ben Lynn blynn@cs.stanford.edu 💡