Swifties

a custom language construction kit

intro

Swifties aims to provide a flexible toolkit for creating custom languages in Swift.

demo

A custom Lisp REPL is provided for demonstration purposes.

primitives

Primitives are called at compile time and may take any number (min/max are specified in the constructor) of forms as arguments and emit operations.

let pos = Pos(source: "test", line: -1, column: -1)

let p = Prim(env: env, pos: self.pos, name: "do", (0, -1), { 
pos, args in
    for a in args { try a.emit() }
})

env.openScope().bind(pos: pos, id: "do", env.coreLib!.primType, p)

functions

Functions may take any number of arguments and return any number of results; pos indicates the call site; evaluation resumes from the returned Pc.

let pos = Pos(source: "test", line: -1, column: -1)

let f = Func(env: env, pos: pos, name: "foo", args: [], rets: [env.coreLib!.intType], {
pos, self, ret in
    env.push(env.coreLib!.intType, 42)
    return ret
})

env.openScope().bind(pos: pos, id: "foo", env.coreLib!.funcType, f)

Functions may alternatively be instantiated with Form-bodies, which emits operations behind the scenes and generates a function containing the code required to evaluate them.

let f = Func(env: env, pos: pos, name: "foo", args: [], rets: [env.coreLib!.intType])
try f.compileBody(LiteralForm(env: env, pos: pos, env.coreLib!.intType, 42))

multimethods

Multimethods are sets of functions sharing the same name sorted from most specific to least that delegate to the most specific applicable function when called.

let m = Multi(env: env, pos: pos, name: "foo")
f.addFunc(f)

types

Two levels of types are used, ÀnyType, and it's direct parameterized subclass Type<T> from which all other types inherit.

• Any - Any kind of value
• Bool - Boolean values
• Char - Character values
• Cont - Continuations as values
• Form - Forms as values
• Func - Functions as values
• Id - Quoted identifiers
• Int - Integer values
• Iter - Iterators
• Macro - Macros as values
• Meta - Types as values
• Multi - Multimethods as values
• Pair - Pairs of values
• Prim - Primitives as values
• Register - Register references as values
• Seq - Iterable values
• Stack - Stack values
• String - String values
• Target - Callable values

parsing

Parser may be used to simplify the process of turning code into forms.

let pos = Pos("test", line: -1, column: -1)
let env = Env()
try env.initCoreLib(pos: pos)

let parser = Parser(env: env, source: "test",
                    prefix: [spaceReader, intReader],
                    suffix: [])
                    
try parser.slurp("1 2 3")
for f in parser.forms { try f.emit() }
env.emit(STOP)
try env.eval(0)

XCTAssertEqual(Slot(env.coreLib!.intType, 3), env.pop(pos: pos)) 
XCTAssertEqual(Slot(env.coreLib!.intType, 2), env.pop(pos: pos)) 
XCTAssertEqual(Slot(env.coreLib!.intType, 1), env.pop(pos: pos)) 

readers

Readers specialize in parsing a specific kind of form.

• Call - Reads call forms
• Char - Reads character literals
• Id - Reads identifiers
• Int - Reads integer literals
• Pair - Reads pair literals
• Quote - Reads quoted forms
• Space - Skips whitespace
• Splice - Reads spliced expressions
• Stack - Reads stack literals
• String - Reads string literals

It's trivial to extend the framework with custom readers. Just make sure to return nil if you can't find what you're looking for, since each reader is tried in sequence for every new position.

func intReader(_ p: Parser) throws -> Form? {
    let fpos = p.pos
    var v = 0
        
    while let c = p.getc() {
        if !c.isNumber {
            p.ungetc(c)
            break
        }
            
        v *= 10
        v += c.hexDigitValue!
        p.nextColumn()
    }
        
    return (p.pos == fpos) ? nil : LiteralForm(env: p.env, pos: p.pos, p.env.coreLib!.intType, v)
}

forms

Code is parsed into forms, which is what primitives and macros operate on.

• Call - Emits code to call specified target with args
• Do - Emits args in sequence
• Id - Emits the value of specified binding and calls it if possible
• Literal - Emits code to push specified value
• Pair - Emits code to push specified pair
• Quote - Emits code to push quoted form
• Splice - Emits result of evaluating form if quoted depth is 1
• Stack - Emits code to push a stack with specified items

operations

Forms emit operations, which are the basic building blocks that are eventually evaluated in sequence to get the desired result.

• Bench - Repeats body specified number of times and pushes elapsed time in milliseconds
• Branch - Branches conditionally
• Call - Calls specified value
• Drop - Drops specified number of items from stack
• For - Repeats code for each value in sequence on top of stack
• Goto - Resumes evaluation from specified Pc
• Load - Loads value from specified register
• Push - Pushes specified value on stack
• PushDown - Pushes top of stack onto next item
• Quote - Quotes and pushes specified form
• Recall - Restarts current function without pushing frame
• Reset - Clears stack
• Restore - Restores continuation
• Return - Pops frame from call stack and resumes evaluation from it's return pc
• Splat - Replaces top of stack (which has to be iterable) with it's items
• Stop - Stops evaluation without error
• Store - Stores value in specified register
• Suspend - Pushes continuation
• Zip - Replaces top two stack items with pair

Operations may be manually emitted at any point using Env.emit(Op).

let pos = Pos("test", line: -1, column: -1)
let env = Env()
try env.initCoreLib(pos: pos)
let v = Slot(env.coreLib!.intType, 42)
env.emit(Push(pc: env.pc, v))
env.emit(STOP)
try env.eval(0)
XCTAssertEqual(v, env.pop(pos: pos))

todo

• add filter like map
  • add odd?/even? to math
• add lambdas
  • extract subclass from Func
  • add type
  • trap stack target in call form
    • push lambda
    • ([x y z] [] ...)
• finish macros
• add support for \n & \t to char/stringReader
• add string interpolation
  • swift syntax
• add unsafe prim
  • add Env.safetyLevel = 0
  • add Unsafe op
    • copy Bench
    • dec/defer inc safetyLevel
  • add Env.unsafe { safetyLevel > 0 }
  • skip Func.IsApplicable if env.unsafe
  • skip result check in Frame.restore if env.unsafe
• add return prim
  • emit args & Return
• make suggestions based on edit distance for missing ids
  • recursive like find