mal/cpp/step5_tco.cpp

#include "MAL.h"

#include "Environment.h"
#include "ReadLine.h"
#include "Types.h"

#include <iostream>
#include <memory>

malValuePtr READ(const String& input);
String PRINT(malValuePtr ast);
static void installFunctions(malEnvPtr env);

static ReadLine s_readLine("~/.mal-history");

static malEnvPtr replEnv(new malEnv);

int main(int argc, char* argv[])
{
    String prompt = "user> ";
    String input;
    installCore(replEnv);
    installFunctions(replEnv);
    while (s_readLine.get(prompt, input)) {
        String out;
        try {
            out = rep(input, replEnv);
        }
        catch (malEmptyInputException&) {
            continue; // no output
        }
        catch (String& s) {
            out = s;
        };
        std::cout << out << "\n";
    }
    return 0;
}

String rep(const String& input, malEnvPtr env)
{
    return PRINT(EVAL(READ(input), env));
}

malValuePtr READ(const String& input)
{
    return readStr(input);
}

malValuePtr EVAL(malValuePtr ast, malEnvPtr env)
{
    if (!env) {
        env = replEnv;
    }
    while (1) {
        const malList* list = DYNAMIC_CAST(malList, ast);
        if (!list || (list->count() == 0)) {
            return ast->eval(env);
        }

        // From here on down we are evaluating a non-empty list.
        // First handle the special forms.
        if (const malSymbol* symbol = DYNAMIC_CAST(malSymbol, list->item(0))) {
            String special = symbol->value();
            int argCount = list->count() - 1;

            if (special == "def!") {
                checkArgsIs("def!", 2, argCount);
                const malSymbol* id = VALUE_CAST(malSymbol, list->item(1));
                return env->set(id->value(), EVAL(list->item(2), env));
            }

            if (special == "do") {
                checkArgsAtLeast("do", 1, argCount);

                for (int i = 1; i < argCount; i++) {
                    EVAL(list->item(i), env);
                }
                ast = list->item(argCount);
                continue; // TCO
            }

            if (special == "fn*") {
                checkArgsIs("fn*", 2, argCount);

                const malSequence* bindings =
                    VALUE_CAST(malSequence, list->item(1));
                StringVec params;
                for (int i = 0; i < bindings->count(); i++) {
                    const malSymbol* sym =
                        VALUE_CAST(malSymbol, bindings->item(i));
                    params.push_back(sym->value());
                }

                return mal::lambda(params, list->item(2), env);
            }

            if (special == "if") {
                checkArgsBetween("if", 2, 3, argCount);

                bool isTrue = EVAL(list->item(1), env)->isTrue();
                if (!isTrue && (argCount == 2)) {
                    return mal::nilValue();
                }
                ast = list->item(isTrue ? 2 : 3);
                continue; // TCO
            }

            if (special == "let*") {
                checkArgsIs("let*", 2, argCount);
                const malSequence* bindings =
                    VALUE_CAST(malSequence, list->item(1));
                int count = checkArgsEven("let*", bindings->count());
                malEnvPtr inner(new malEnv(env));
                for (int i = 0; i < count; i += 2) {
                    const malSymbol* var =
                        VALUE_CAST(malSymbol, bindings->item(i));
                    inner->set(var->value(), EVAL(bindings->item(i+1), inner));
                }
                ast = list->item(2);
                env = inner;
                continue; // TCO
            }
        }

        // Now we're left with the case of a regular list to be evaluated.
        std::unique_ptr<malValueVec> items(list->evalItems(env));
        malValuePtr op = items->at(0);
        if (const malLambda* lambda = DYNAMIC_CAST(malLambda, op)) {
            ast = lambda->getBody();
            env = lambda->makeEnv(items->begin()+1, items->end());
            continue; // TCO
        }
        else {
            return APPLY(op, items->begin()+1, items->end());
        }
    }
}

String PRINT(malValuePtr ast)
{
    return ast->print(true);
}

malValuePtr APPLY(malValuePtr op, malValueIter argsBegin, malValueIter argsEnd)
{
    const malApplicable* handler = DYNAMIC_CAST(malApplicable, op);
    MAL_CHECK(handler != NULL,
              "\"%s\" is not applicable", op->print(true).c_str());

    return handler->apply(argsBegin, argsEnd);
}

static const char* malFunctionTable[] = {
    "(def! list (fn* (& items) items))",
    "(def! not (fn* (cond) (if cond false true)))",
    "(def! >= (fn* (a b) (<= b a)))",
    "(def! < (fn* (a b) (not (<= b a))))",
    "(def! > (fn* (a b) (not (<= a b))))",
};

static void installFunctions(malEnvPtr env) {
    for (auto &function : malFunctionTable) {
        rep(function, env);
    }
}

// Added to keep the linker happy at step A
malValuePtr readline(const String& prompt)
{
    String input;
    if (s_readLine.get(prompt, input)) {
        return mal::string(input);
    }
    return mal::nilValue();
}