README.md

@@ -5,23 +5,41 @@ Specification and discussions about language design.
 A toy implementation is in [`toy-impl`](./toy-impl), requires python 3.9.
 
 
-## Referencing Python Variables
-The kernel is allowed to read host Python variables, but has to specify with
-`global` before referencing them. This is to simplify and speed-up
-implementation, and also warn the user about the variable being global. (prevent
-calling the interpreter many times during compilation if there are many
-references to host variables)
+## Referencing Host Variables from Kernel
+Host variable to be accessed must be declared as `global` in the kernel
+function.  This is to simplify and speed-up implementation, and also warn the
+user about the variable being global. (prevent calling the interpreter many
+times during compilation if there are many references to host variables)
 
-Only primitive types and tuple/list of primitive types are allowed.
+Kernel cannot modify host variables, this would be checked by the compiler.
+Value that can be observed by the kernel would be frozen once the kernel has
+been compiled, subsequence modification within the host would not affect the
+kernel.
 
-The value would be substituted at compile time, subsequent modification in the
-host would not be known by the kernel.
+Only types supported in the kernel can be referenced.
 
-Modification in kernel code to the global variables is not allowed.
+Examples:
+```python
+FOO = 0
+
+@kernel
+def correct() -> int:
+    global FOO
+    return FOO + 1
+
+@kernel
+def fail_without_global() -> int:
+    return FOO + 2
+
+@kernel
+def fail_write() -> None:
+    FOO += 1
+
+```
 
 ## Class and Functions
-* Class fields must be annotated:
-    ```py
+* Instance variables must be annotated: (Issue #1)
+    ```python
     class Foo:
         a: int
         b: int
@@ -29,24 +47,26 @@ Modification in kernel code to the global variables is not allowed.
             self.a = a
             self.b = b
     ```
+* Three types of instance variables: (Issue #5)
+  * Host only variables: Do not add type annotation for it in the class.
+  * Kernel Invariants: Immutable in the kernel and in the host while the kernel
+    is executing. Type: `KernelInvariant[T]`. The types must be immutable.
+    (use tuple instead of list in the host, but the type annotation should still
+    be list?)
+  * Normal Variables: The host can only assign to them in the `__init__`
+    function. Not accessible afterwards.
 * Functions require full type signature, including type annotation to every
   parameter and return type.
-  ```py
+  ```python
   def add(a: int, b: int) -> int:
     return a + b
   ```
-* No implicit coercion, require implicit cast. Integers are int32 by default,
-  floating point numbers are double by default.
 * RPCs: optional parameter type signature, require return type signature.
-* Function default parameters are not allowed, as changes to the default value
-  would not be kept across kernel calls, which is a potential source of
-  confusion. (maybe we can allow primitive default types?)
-* Cannot construct objects within kernel code.
+* Function default parameters must be immutable.
+* Function pointers are supported,  and lambda expression is not supported
+  currently. (maybe support lambda after implementing type inference?)
 
-Questions:
-* Can we construct objects within kernel code?
-* Should we support function pointers? What about subtyping with function
-  pointers, and generic types?
+  Its type is denoted by the typing library, e.g. `Call[[int32, int32], int32]`.
 
 ## Built-in Types
 * Primitive types include:
@@ -57,14 +77,14 @@ Questions:
     * `uint32`
     * `uint64`
     * `float`
-    * `str` (note: fixed length, provide builtin methods?)
-    * `bytes` (a list of `byte`, but with more convenient syntax)
+    * `str`
+    * `bytes`
 * Collections include:
     * `list`: homogeneous (elements must be of the same type) fixed-size (no
       append) list.
-    * `tuple`: inhomogeneous fixed-size list, only pattern
-      matching (e.g. `a, b, c = (1, True, 1.2)`) and constant indexing
-      is supported:
+    * `tuple`: inhomogeneous immutable list, only pattern
+      matching (e.g. `a, b, c = (1, True, 1.2)`) and constant indexing is
+      supported:
       ```
       t = (1, True)
       # OK
@@ -75,23 +95,34 @@ Questions:
       i = 0
       a = t[i]
       ```
-    * `range` (over numerical types) (not sure if this is really useful)
+    * `range` (over numerical types)
 
 ### Numerical Types
-* All binary operations expect the values to have the same type, no implicit
-  coercion would be performed, explicit casting is required.
+* All binary operations expect the values to have the same type.
 * Casting can be done by `T(v)` where `T` is the target type, and `v` is the
   original value. Examples: `int64(123)`
-* Constant integers are treated as `int32` by default. If the value cannot
-  be stored in `int32`, `uint64` would be used if the integer is non-negative,
-  and `int64` would be used it the integer is negative.
+* Integers are treated as `int32` by default. Floating point numbers are double
+  by default.
+* No implicit coercion, require implicit cast.
+  For integers that don't fit in int32, users should cast them to `int64`
+  explicitly, i.e. `int64(2147483648)`.  If the compiler found that the integer
+  does not fit into int32, it would raise an error. (Issue #2)
 * Only `uint32`, `int32` (and range of them) can be used as index.
 
+### Kernel Only class
+* Annotate the class with `@kernel`/`@portable`.
+* The instance can be created from within kernel functions, or the host if it is
+  portable. It can be passed into kernels.
+* All methods, including the constructor, are treated as kernel/portable
+  functions that would be compiled by the compiler, no RPC function is allowed.
+* If the instance is passed into the kernel, the host is not allowed to access
+  the instance data. Access would raise exception.
+
 ## Generics
 We use [type variable](https://docs.python.org/3/library/typing.html#typing.TypeVar) for denoting generics.
 
 Example:
-```py
+```python
 from typing import TypeVar
 T = TypeVar('T')
 
@@ -102,16 +133,13 @@ class Foo(EnvExperiment):
         return a == b
 ```
 
-* Type variables can only be used in functions/methods, but not in classes.
-    (this can be relaxed, only allow those with type variables fully defined
-    from the constructor)
 * Type variable can be limited to a fixed set of types.
 * Type variables are invariant, same as the default in Python. We disallow
   covariant or contravariant. The compiler should mark as error if it encounters
   a type variable used in kernel that is declared covariant or contravariant.
 * Code region protected by a type check, such as `if type(x) == int:`, would
   treat `x` as `int`, similar to how [typescript type guard](https://www.typescripttutorial.net/typescript-tutorial/typescript-type-guards/) works.
-  ```py
+  ```python
   def add1(x: Union[int, bool]) -> int:
     if type(x) == int:
         # x is int
@@ -126,53 +154,6 @@ class Foo(EnvExperiment):
 * Type variable cannot occur alone in the result type, i.e. must be bound to the
   input parameters.
 
-Questions:
-* Should we support things like optional type? (like the one in rust)
-* Would it be better to assert on the type variable directly instead of
-  `type(x)` for type guards?
-
-## Dynamic Dispatch
-Type annotations are invariant, so subtype (derived types) cannot be used
-when the base type is expected. Example:
-```py
-class Base:
-    def foo(self) -> int:
-        return 1
-
-class Derived(Base):
-    def foo(self) -> int:
-        return 2
-
-def bar(x: list[Base]) -> int:
-    sum = 0
-    for v in x:
-        sum += v.foo()
-    return sum
-
-# incorrect, this list cannot be typed (inhomogeneous)
-bar([Base(), Derived()])
-```
-
-Dynamic dispatch is supported, but requires explicit annotation, similar to
-[trait object](https://doc.rust-lang.org/book/ch17-02-trait-objects.html) in rust.
-`virtual[T]` is the type for `T` and its subtypes(derived types).
-
-This is mainly for performance consideration, as virtual method table that is
-required for dynamic dispatch would penalize performance, and prohibits function
-inlining etc.
-
-Type variables cannot be used inside `virtual[...]`, and type variables would not
-range over `virtual[...]`.
-
-> Not sure what is the best syntax for `virtual[...]`
-
-Example:
-```py
-def bar2(x: list[virtual[Base]]) -> int:
-    sum = 0
-    for v in x:
-        sum += v.foo()
-    return sum
-```
-
+## Lifetime
+Probably need more discussions...