mal/swift/step5_tco.swift

//******************************************************************************
// MAL - step 5 - tco
//******************************************************************************
// This file is automatically generated from templates/step.swift. Rather than
// editing it directly, it's probably better to edit templates/step.swift and
// regenerate this file. Otherwise, your change might be lost if/when someone
// else performs that process.
//******************************************************************************

import Foundation

// The number of times EVAL has been entered recursively. We keep track of this
// so that we can protect against overrunning the stack.
//
private var EVAL_level = 0

// The maximum number of times we let EVAL recurse before throwing an exception.
// Testing puts this at some place between 1800 and 1900. Let's keep it at 500
// for safety's sake.
//
private let EVAL_leval_max = 500

// Control whether or not tail-call optimization (TCO) is enabled. We want it
// `true` most of the time, but may disable it for debugging purposes (it's
// easier to get a meaningful backtrace that way).
//
private let TCO = true

// Control whether or not we emit debugging statements in EVAL.
//
private let DEBUG_EVAL = false

// String used to prefix information logged in EVAL. Increasing lengths of the
// string are used the more EVAL is recursed.
//
private let INDENT_TEMPLATE = "|----|----|----|----|----|----|----|----|" +
    "----|----|----|----|----|----|----|----|----|----|----|" +
    "----|----|----|----|----|----|----|----|----|----|----|" +
    "----|----|----|----|----|----|----|----|----|----|----|" +
    "----|----|----|----|----|----|----|----|----|----|----|" +
    "----|----|----|----|----|----|----|----|----|----|----|" +
    "----|----|----|----|----|----|----|----|----|----|----|" +
    "----|----|----|----|----|----|----|----|----|----|----|" +
    "----|----|----|----|----|----|----|----|----|----|----|" +
    "----|----|----|----|----|----|----|----|----|----|----|" +
    "----|----|----|----|----|----|----|----|----|----|----|"

// Holds the prefix of INDENT_TEMPLATE used for actual logging.
//
private var indent = String()

// Symbols used in this module.
//
private let kValDef           = make_symbol("def!")
private let kValDo            = make_symbol("do")
private let kValFn            = make_symbol("fn*")
private let kValIf            = make_symbol("if")
private let kValLet           = make_symbol("let*")
private let kValTry           = make_symbol("try*")

private let kSymbolDef           = as_symbol(kValDef)
private let kSymbolDo            = as_symbol(kValDo)
private let kSymbolFn            = as_symbol(kValFn)
private let kSymbolIf            = as_symbol(kValIf)
private let kSymbolLet           = as_symbol(kValLet)

func substring(s: String, _ begin: Int, _ end: Int) -> String {
    return s[s.startIndex.advancedBy(begin) ..< s.startIndex.advancedBy(end)]
}

// Parse the string into an AST.
//
private func READ(str: String) throws -> MalVal {
    return try read_str(str)
}

// Perform a simple evaluation of the `ast` object. If it's a symbol,
// dereference it and return its value. If it's a collection, call EVAL on all
// elements (or just the values, in the case of the hashmap). Otherwise, return
// the object unchanged.
//
private func eval_ast(ast: MalVal, _ env: Environment) throws -> MalVal {
    if let symbol = as_symbolQ(ast) {
        guard let val = env.get(symbol) else {
            try throw_error("'\(symbol)' not found")    // Specific text needed to match MAL unit tests
        }
        return val
    }
    if let list = as_listQ(ast) {
        var result = [MalVal]()
        result.reserveCapacity(Int(list.count))
        for item in list {
            let eval = try EVAL(item, env)
            result.append(eval)
        }
        return make_list(result)
    }
    if let vec = as_vectorQ(ast) {
        var result = [MalVal]()
        result.reserveCapacity(Int(vec.count))
        for item in vec {
            let eval = try EVAL(item, env)
            result.append(eval)
        }
        return make_vector(result)
    }
    if let hash = as_hashmapQ(ast) {
        var result = [MalVal]()
        result.reserveCapacity(Int(hash.count) * 2)
        for (k, v) in hash {
            let new_v = try EVAL(v, env)
            result.append(k)
            result.append(new_v)
        }
        return make_hashmap(result)
    }
    return ast
}

private enum TCOVal {
    case NoResult
    case Return(MalVal)
    case Continue(MalVal, Environment)

    init() { self = .NoResult }
    init(_ result: MalVal) { self = .Return(result) }
    init(_ ast: MalVal, _ env: Environment) { self = .Continue(ast, env) }
}

// EVALuate "def!".
//
private func eval_def(list: MalSequence, _ env: Environment) throws -> TCOVal {
    guard list.count == 3 else {
        try throw_error("expected 2 arguments to def!, got \(list.count - 1)")
    }
    let arg1 = try! list.nth(1)
    let arg2 = try! list.nth(2)
    guard let sym = as_symbolQ(arg1) else {
        try throw_error("expected symbol for first argument to def!")
    }
    let value = try EVAL(arg2, env)
    return TCOVal(env.set(sym, value))
}

// EVALuate "let*".
//
private func eval_let(list: MalSequence, _ env: Environment) throws -> TCOVal {
    guard list.count == 3 else {
        try throw_error("expected 2 arguments to let*, got \(list.count - 1)")
    }
    let arg1 = try! list.nth(1)
    let arg2 = try! list.nth(2)
    guard let bindings = as_sequenceQ(arg1) else {
        try throw_error("expected list for first argument to let*")
    }
    guard bindings.count % 2 == 0 else {
        try throw_error("expected even number of elements in bindings to let*, got \(bindings.count)")
    }
    let new_env = Environment(outer: env)
    for var index: MalIntType = 0; index < bindings.count; index += 2 {
        let binding_name = try! bindings.nth(index)
        let binding_value = try! bindings.nth(index + 1)
        guard let binding_symbol = as_symbolQ(binding_name) else {
            try throw_error("expected symbol for first element in binding pair")
        }
        let evaluated_value = try EVAL(binding_value, new_env)
        new_env.set(binding_symbol, evaluated_value)
    }
    if TCO {
        return TCOVal(arg2, new_env)
    }
    return TCOVal(try EVAL(arg2, new_env))
}

// EVALuate "do".
//
private func eval_do(list: MalSequence, _ env: Environment) throws -> TCOVal {
    if TCO {
        let _ = try eval_ast(list.range_from(1, to: list.count-1), env)
        return TCOVal(list.last(), env)
    }

    let evaluated_ast = try eval_ast(list.rest(), env)
    let evaluated_seq = as_sequence(evaluated_ast)
    return TCOVal(evaluated_seq.last())
}

// EVALuate "if".
//
private func eval_if(list: MalSequence, _ env: Environment) throws -> TCOVal {
    guard list.count >= 3 else {
        try throw_error("expected at least 2 arguments to if, got \(list.count - 1)")
    }
    let cond_result = try EVAL(try! list.nth(1), env)
    var new_ast: MalVal
    if is_truthy(cond_result) {
        new_ast = try! list.nth(2)
    } else if list.count == 4 {
        new_ast = try! list.nth(3)
    } else {
        return TCOVal(make_nil())
    }
    if TCO {
        return TCOVal(new_ast, env)
    }
    return TCOVal(try EVAL(new_ast, env))
}

// EVALuate "fn*".
//
private func eval_fn(list: MalSequence, _ env: Environment) throws -> TCOVal {
    guard list.count == 3 else {
        try throw_error("expected 2 arguments to fn*, got \(list.count - 1)")
    }
    guard let seq = as_sequenceQ(try! list.nth(1)) else {
        try throw_error("expected list or vector for first argument to fn*")
    }
    return TCOVal(make_closure((eval: EVAL, args: seq, body: try! list.nth(2), env: env)))
}

// Walk the AST and completely evaluate it, handling macro expansions, special
// forms and function calls.
//
private func EVAL(var ast: MalVal, var _ env: Environment) throws -> MalVal {
    EVAL_level++
    defer { EVAL_level-- }
    guard EVAL_level <= EVAL_leval_max else {
        try throw_error("Recursing too many levels (> \(EVAL_leval_max))")
    }

    if DEBUG_EVAL {
        indent = substring(INDENT_TEMPLATE, 0, EVAL_level)
    }

    while true {
        if DEBUG_EVAL { print("\(indent)>   \(ast)") }

        if !is_list(ast) {

            // Not a list -- just evaluate and return.

            let answer = try eval_ast(ast, env)
            if DEBUG_EVAL { print("\(indent)>>> \(answer)") }
            return answer
        }

        // Special handling if it's a list.

        let list = as_list(ast)
        if DEBUG_EVAL { print("\(indent)>.  \(list)") }

        if list.isEmpty {
            return ast
        }

        // Check for special forms, where we want to check the operation
        // before evaluating all of the parameters.

        let arg0 = list.first()
        if let fn_symbol = as_symbolQ(arg0) {
            let res: TCOVal

            switch fn_symbol {
                case kSymbolDef:            res = try eval_def(list, env)
                case kSymbolLet:            res = try eval_let(list, env)
                case kSymbolDo:             res = try eval_do(list, env)
                case kSymbolIf:             res = try eval_if(list, env)
                case kSymbolFn:             res = try eval_fn(list, env)
                default:                    res = TCOVal()
            }
            switch res {
                case let .Return(result):               return result
                case let .Continue(new_ast, new_env):   ast = new_ast; env = new_env; continue
                case .NoResult:                         break
            }
        }

        // Standard list to be applied. Evaluate all the elements first.

        let eval = try eval_ast(ast, env)

        // The result had better be a list and better be non-empty.

        let eval_list = as_list(eval)
        if eval_list.isEmpty {
            return eval
        }

        if DEBUG_EVAL { print("\(indent)>>  \(eval)") }

        // Get the first element of the list and execute it.

        let first = eval_list.first()
        let rest = as_sequence(eval_list.rest())

        if let fn = as_builtinQ(first) {
            let answer = try fn.apply(rest)
            if DEBUG_EVAL { print("\(indent)>>> \(answer)") }
            return answer
        } else if let fn = as_closureQ(first) {
            let new_env = Environment(outer: fn.env)
            let _ = try new_env.set_bindings(fn.args, with_exprs: rest)
            if TCO {
                env = new_env
                ast = fn.body
                continue
            }
            let answer = try EVAL(fn.body, new_env)
            if DEBUG_EVAL { print("\(indent)>>> \(answer)") }
            return answer
        }

        // The first element wasn't a function to be executed. Return an
        // error saying so.

        try throw_error("first list item does not evaluate to a function: \(first)")
    }
}

// Convert the value into a human-readable string for printing.
//
private func PRINT(exp: MalVal) -> String {
    return pr_str(exp, true)
}

// Perform the READ and EVAL steps. Useful for when you don't care about the
// printable result.
//
private func RE(text: String, _ env: Environment) -> MalVal? {
    if !text.isEmpty {
        do {
            let ast = try READ(text)
            do {
                return try EVAL(ast, env)
            } catch let error as MalException {
                print("Error evaluating input: \(error)")
            } catch {
                print("Error evaluating input: \(error)")
            }
        } catch let error as MalException {
            print("Error parsing input: \(error)")
        } catch {
            print("Error parsing input: \(error)")
        }
    }
    return nil
}

// Perform the full READ/EVAL/PRINT, returning a printable string.
//
private func REP(text: String, _ env: Environment) -> String? {
    let exp = RE(text, env)
    if exp == nil { return nil }
    return PRINT(exp!)
}

// Perform the full REPL.
//
private func REPL(env: Environment) {
    while true {
        if let text = _readline("user> ") {
            if let output = REP(text, env) {
                print("\(output)")
            }
        } else {
            print("")
            break
        }
    }
}

func main() {
    let env = Environment(outer: nil)

    load_history_file()
    load_builtins(env)

    RE("(def! not (fn* (a) (if a false true)))", env)
    REPL(env)

    save_history_file()
}