In FunC (and in Tolk before) throwing an exception is just
calling a built-in function:
> throw 123; // actually, __throw(123)
Since it's a regular function, the compiler was not aware
that execution will stop, and all following code is unreachable.
For instance, `throw` in the end on function needed to be
followed by `return` statement.
Now, `throw` interrupts control flow, all statements after
it are considered unreachable. At IR level, code Ops are
also not produced.
This works because a built-in __throw() now has `never` type.
It can also be applied to custom functions:
> fun alwaysThrow(): never { throw 123; }
The code after alwaysThrow() call will also be unreachable.
With the introduction of nullable types, we want the
compiler to be smart in cases like
> if (x == null) return;
> // x is int now
or
> if (x == null) x = 0;
> // x is int now
These are called smart casts: when the type of variable
at particular usage might differ from its declaration.
Implementing smart casts is very challenging. They are based
on building control-flow graph and handling every AST vertex
with care. Actually, I represent cfg not a as a "graph with
edges". Instead, it's a "structured DFS" for the AST:
1) at every point of inferring, we have "current flow facts"
2) when we see an `if (...)`, we create two derived contexts
3) after `if`, finalize them at the end and unify
4) if we detect unreachable code, we mark that context
In other words, we get the effect of a CFG but in a more direct
approach. That's enough for AST-level data-flow.
Smart casts work for local variables and tensor/tuple indices.
Compilation errors have been reworked and now are more friendly.
There are also compilation warnings for always true/false
conditions inside if, assert, etc.
This commit introduces nullable types `T?` that are
distinct from non-nullable `T`.
Example: `int?` (int or null) and `int` are different now.
Previously, `null` could be assigned to any primitive type.
Now, it can be assigned only to `T?`.
A non-null assertion operator `!` was also introduced,
similar to `!` in TypeScript and `!!` in Kotlin.
If `int?` still occupies 1 stack slot, `(int,int)?` and
other nullable tensors occupy N+1 slots, the last for
"null precedence". `v == null` actually compares that slot.
Assigning `(int,int)` to `(int,int)?` implicitly creates
a null presence slot. Assigning `null` to `(int,int)?` widens
this null value to 3 slots. This is called "type transitioning".
All stdlib functions prototypes have been updated to reflect
whether they return/accept a nullable or a strict value.
This commit also contains refactoring from `const FunctionData*`
to `FunctionPtr` and similar.
In FunC (and in Tolk before), the assignment
> lhs = rhs
evaluation order (at IR level) was "rhs first, lhs second".
In practice, this did not matter, because lhs could only
be a primitive:
> (v1, v2) = getValue()
Left side of assignment actually has no "evaluation".
Since Tolk implemented indexed access, there could be
> getTensor().0 = getValue()
or (in the future)
> getObject().field = getValue()
where evaluation order becomes significant.
Now evaluation order will be to "lhs first, rhs second"
(more expected from user's point of view), which will become
significant when building control flow graph.
It works both for reading and writing:
> var t = (1, 2);
> t.0; // 1
> t.0 = 5;
> t; // (5, 2)
It also works for typed/untyped tuples, producing INDEX and SETINDEX.
Global tensors and tuples works. Nesting `t.0.1.2` works. `mutate` works.
Even mixing tuples inside tensors inside a global for writing works.
Currently, tolk-tester can test various "output" of the compiler:
pass input and check output, validate fif codegen, etc.
But it can not test compiler internals and AST representation.
I've added an ability to have special functions to check/expose
internal compiler state. The first (and the only now) is:
> __expect_type(some_expr, "<type>");
Such a call has special treatment in a compilation process.
Compilation fails if this expression doesn't have requested type.
It's intended to be used in tests only. Not present in stdlib.
Comparison operators `== / >= /...` return `bool`.
Logical operators `&& ||` return bool.
Constants `true` and `false` have the `bool` type.
Lots of stdlib functions return `bool`, not `int`.
Operator `!x` supports both `int` and `bool`.
Condition of `if` accepts both `int` and `bool`.
Arithmetic operators are restricted to integers.
Logical `&&` and `||` accept both `bool` and `int`.
No arithmetic operations with bools allowed (only bitwise and logical).
FunC's (and Tolk's before this PR) type system is based on Hindley-Milner.
This is a common approach for functional languages, where
types are inferred from usage through unification.
As a result, type declarations are not necessary:
() f(a,b) { return a+b; } // a and b now int, since `+` (int, int)
While this approach works for now, problems arise with the introduction
of new types like bool, where `!x` must handle both int and bool.
It will also become incompatible with int32 and other strict integers.
This will clash with structure methods, struggle with proper generics,
and become entirely impractical for union types.
This PR completely rewrites the type system targeting the future.
1) type of any expression is inferred and never changed
2) this is available because dependent expressions already inferred
3) forall completely removed, generic functions introduced
(they work like template functions actually, instantiated while inferring)
4) instantiation `<...>` syntax, example: `t.tupleAt<int>(0)`
5) `as` keyword, for example `t.tupleAt(0) as int`
6) methods binding is done along with type inferring, not before
("before", as worked previously, was always a wrong approach)
