SLUL2
=====

Making SLUL:
* easier to use
* easier to implement
* more future-proof / more portable

Desirable changes:

* Revise ref
    - forbidding refs to non-struct/non-array types might enable some optimizations
    - *removing* explicit refs would definitely have impacts on usability.
      not sure if good or bad. it makes the proglang more implicit/"magic",
      which can be a bad thing.
* Implicit arenas?
    - and for mutating methods, use the 4 "allocation variants": (inspired by Vale)
        1. placement new        (uses arena only for "indirect"/references fields)
        2. arena new            (allocate in given arena)
        3. modify self          (in-place modification. Can keep referenced data)
        4. discarding self-mod  (in-place modification. Cannot keep referenced data)
        some of these are applicable to constructors also
* Garbage-collected areanas?
    - I.e. local GC
    - It would remove "anxiety" around memory allocation
    - Downside 1: It requires meta-data, which isn't needed with plain
      arena allocation.
    - Downsides N: The usual downsides with GC
* Revise expr integer types
  What are the use-cases for "non-plain ints"?
    - length type / ssize/usize
    - byte/int16 arrays
    - small/bitsliced fields in structs
    - fixed-size wrapping uints (e.g. for hash functions).
* Move some stuff from hard-coded syntax to code? e.g. like Scheme, REBOL, Nim.
    - might actually work with statements:
        - they can only appear inside function bodies
        - so the toplevels are available, and their types are known.
    - But it would definitely need inlining to work with reasonable speed.
    - Perhaps a bad idea after all?
* Misc syntax stuff:
    - Use tabs instead of spaces?
      But this gets tricky with alignment of e.g. parameters.


Control statements defined in library module headers
----------------------------------------------------

These need some kind of analysis of the IR to check varstates (liveness, etc).
It also needs to handle nested if-elseif-elseif...

So perhaps this is a bad idea?

Example:

    statement "if" Expr cond Statements true_block "else" Statements false_block
    {
        cond
        CONDJUMP FALSE false_block
        true_block
        JUMP end
        false_block
        end
    }
    statement

Super-simple proglang
---------------------

Only two/three kinds of typedefs. Not allowed as anonymous types (or? it is useful for e.g. return values)

    record SomeStruct {
        int field1
        OtherType field2    # <--- compiler chooses whether to put in ref or not
                            # this makes FFI trickier. But non-closed types are always refs.
                            # this also makes lifetimes and aliasing trickier.
                            # maybe "var" should not be allowed to alias?
    }

    enum SomeEnum {
        ...
    }

    # Maybe some kind of sum/union/variant type
    record ExprNode {
        ExprType type
        int line
        int column
        switch type {
        case .unary
        case .binary
            Expr operand_a
            if type == .binary {
                Expr operand_b
            }
        case .call
            Expr func_expr
            int num_args
            # have a built-in list type?
            # and choose the best possible representation?
            # (in this case it's runtime-determined frozen-length, so it could be a pointer to an array. or a full-blown list type)
            int[num_args] args
        }
    }

    # Maybe some kind of constraints
    func process_op(ExprNode<.type in (.unary, .binary)> expr)
    func process_op(ExprNode expr [.unary, .binary])
    func process_op(ExprNode(.unary .binary) expr)
    func process_op(ExprNode<.unary .binary> expr)
    func ExprNode<.unary .binary>.process_op()
    func ExprNode.process_op()
        for (.unary, .binary)
    func ExprNode.process_op()
        with (.unary, .binary)
    func ExprNode.process_op()
        this in (.unary, .binary)
    func ExprNode.process_op()
        given type == .unary or type == .binary

Qualifiers for records and enums:

    record Point closed {  # (require a newline here?)
        int x
        int y
        # no more fields can be added. allows some optimizations, such as call-by-value / embedding into structs
    }

    enum SubPixel closed {
        .red
        .green
        .blue
    }

Enums can have a base type and/or integral values also
(this is mainly useful for FFI)

    enum StatusByte closed byte {
        .ready = 10
        .running = 20
        .failure = 90
    }

Integer / elementary types:

* Perhaps even use variable-size integers?
  The downside is that += 1 etc. might require allocation.

Methods:

* Skip "this". But disallow shadowing.

Type identifiers

* For consistency, always include the "." in typeidentifiers, even in
  e.g. enum definitions.
* Constructors are maybe not that intuitive (can they be improved?):

    func .new(int a, int b) -> Thing


Avoiding punctuation:

* Can the . in typeidentifiers be skipped?
* Can the () in function calls be skipped?
    - if the function call fits on one line
    - (unless a comma is required between them) and the parameters are terms
    - and the function call is not nested inside
      a function call, field or index expression.
    - related: tuples. but that would be ambiguouos if used as function arguments
* Can the () in function declarations be skipped?

    func example
        int a
        int b
        return bool
    {
        if a == b {
            otherfunc a, 123
            return true
        }
        
    }

Can refs be avoided?

    # objects:
    # These are always passed by reference.
    # References can be compared with "ref_is" or "is" or a similar operator.
    # The "==" and "!=" operators are not allowed (maybe it should be allowed to implement them? e.g. with a method called "equals"?)
    type Box = object {
        # These are references:
        Item a
        Item b
        # Perhaps allow syntax like this:
        Item a1, b1
        Item a1, Item b1
        # Regarding tuples:
        # I think that maybe they CAN be references if it too large to use values :)
        # - We can require that if the object is mutable, it must also be passed by arena-ref.
        # - Tuples up to some certain size could be embedded / passed by value
        #   (Check the optimal limit. It's at least the size of two pointers, but it could be larger)
        # - Tuples allocated in the *same* arena can just be referenced directly!
        #   (this should be fairly simple and fast to check).
        #   - If each thread uses a contiguous virtual-memory block,
        #     then this would be a trivial range check.
        # - Tuples allocated by the same thread and in SLUL code, can
        #   (as an optional optimization) be referenced if
        #   1) the lifetime allows it (how to check this at runtime?), or
        #   2) the runtime uses garbage collection, and can perform GC in
        #   this case.
        # - Tuples allocated in SLUL code from other threads may or may not
        #   be possible to reference depending on whether the runtime
        #   supports cross-thread GC. For consistency accross implementations,
        #   it might be better to just re-allocate/copy in this case.
        # - Other tuples would require a copy. (This is really a requirement
        #   for tuples allocated from C code, unless it uses SLUL's arena
        #   allocation functions in slulrt.)
        #   
        LargeValue large
    }
    # opaque objects:
    # - Like objects, but fields (and layout/size) are inaccessible
    # - Lacks {} and has the layout defined in the impl, just like a function can have it's body in the impl
    # - Perhaps it should be forbidden to have non-opaque objects in interfaces? It's generally an anti-pattern.
    type Item = object
    # tuples:
    # - The ABI decides when to pass these by ref or value
    # - Reference comparison operators are not allowed.
    # - The contents can be compared with the "==" and "!=" operators.
    # - Tuples can't be opaque/private.
    type Point = (int x, int y)
    type Point = (int x, y) # perhaps allow this syntax as well (...and multi-line syntax without comma also)
    type LargeValue = ([10000]byte buffer)
    # For type-scoped functions that return an object ("constructors"):
    # - They implicitly take an arena parameter
    # - The returned reference is an arena reference
    func .new() -> Box
    constructor Box.new()   # maybe type-scoped functions should have this syntax?
    # Return values in methods have the same lifetime as the object itself
    # - Should the this parameter be "var"? 
    # - Should the this parameter be "arena"? 
    func Box.get_contents() -> Item
    # Parameters do not implicitly transfer ownership. Inside the callee, the lifetime of "other" ends when the function returns.
    # - Should the parameter be "arena"? 
    func Box.equals(Box other) -> bool
    # Parameters can be marked with "keep" to allow shared ownership
    func Box.set_contents(keep Item contents)
    func Box.set_contents!(keep Item contents)    # perhaps there should be a ! for functions that modify the object?
    func var Box.set_contents(keep Item contents) # or a qualifier like this.
    # Parameters can be passed as "var"
    # - if passed as "keep", we need exclusive access (or the item can be marked as aliased)
    fucn Box.squeeze_item(keep var Item item)
    fucn Box.squeeze_item(keep aliased var Item item)
    # To have mulitple outputs from a function, use a tuple as the return value:
    # - The ABI decides when to pass these by ref (implicit parameter) or value
    # - Because tuples can't be opaque, the return value could be returned by value.
    # - Because tuples can't be opaque, the *caller* allocates (on stack) if it's not possible to pass by value.
    func Box.get_both_items() -> (Item a, Item b)

    # How should function references work?
    # - What keyword to use when there are no refs?
    # - Most of the time, you want a context-parameter
    # - For non-ref (or slot) types, you may want a (reference, length) to process multiple items at once.
    func Box.process_contents(delegate(Item item) handler)

Should the builtin types use TitleCase names also?

    Probably yes, for consistency.

    String
    Byte
    Int16

    Java developers might confuse these with reference types, though :(
    And worse, as a Java developer, you might start using e.g. Byte
    where it should be byte in your Java code. That will usually silently
    compile without any warnings, but can be broken (with == != operators)
    or slow.

    Solution:

    Use different names:
    - byte -> UInt8 or U8
    - int  -> Integer (or even skip this type, and have only fixed-sized Int*/UInt*)

    Regarding the extra finger strain to hold shift and stretching out the
    finger to push the letter button: That could be solved by having the IDE
    auto-capitalize the type if it exists and a type is expected at the given
    location.


Should/can there be a arbitrary-sized integer type?

    E.g. allow integers -16384..16383 to be stored directly, and use a
    reference for larger integers.

    What should it be called?

        num
        int
        intn
        integer
        BigNum
        BigInt
        Num
        Int
        IntN
        Integer

    The compiler could optimize it to a more efficient type if the range
    is known!

        num i = get_number()  # since it is immutable, we can infer the type from the return value of get_number()

    Maybe it should be possible to specify a range? What syntax to use?

        var num<0..=10> i = 0
        var num<0 upto 10> i = 0
        var num i [[0 <= value <= 10]] = 0
        var num<0-10> i = 0     # but "-" is also the minus operator :(
        var num<0~10> i = 0

Print function:

    How simple can it be, without creating confusion/problems or hard-coding things?

        out.print("number: {}", .[123])     # array constructor
        out.print("number: {}", 123)        # safe variant-type var-arg
        out.print "number: {}", 123         # allowing () to be skipped (in some cases)
        out "number: {}", 123               # allowing a default function on objects
        out("number: {}", 123)              # allowing a default function on objects, but without allowing () to be skipped
        # error handling?

    Input streams could also have a default function.
    But it would be limited to only reading e.g. a line.
    (That's probably what iterators should do as well.)
    What should it do on error?

        string s = in()     # reads a line