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This repository is abandoned due to the overwhelming complexity of metaprogramming with Boost/Preprocessor. See Metalang99 and Datatype99 -- the successors.

poica

CI Version Roadmap

The goal of this project is to implement the features of modern programming languages in plain C11 via its macro system, thereby improving static reasoning and achieving a better way to organise our code.

Table of contents

Installation

The recommended way is using Git submodules. Enter your project and execute:

git submodule add -b master https://github.com/Hirrolot/poica
git submodule update --init --recursive

If you use CMake, you can then connect poica as follows:

include_directories(poica/include poica/preprocessor/include poica/vmd/include)

And #include <poica.h> inside your source files to export its public API. Building isn't required, because poica is a header-only library.

If you use GCC, -ftrack-macro-expansion=0 would reduce compilation time and memory consumption.

Algebraic data types

Usually in C we use unions to tell a compiler that we're going to interpret a single memory region in different ways. To decide how to interpret a union, we endow it with a tag and get a tagged union.

However, there'll be quite lot of duplication in code:

typedef struct {
    enum {
        OUR_TAGGED_UNION_STATE_1,
        OUR_TAGGED_UNION_STATE_2,
        OUR_TAGGED_UNION_STATE_3,
    } state;

    union {
        int state_1;
        const char *state_2;
        double state_3;
    } data;
} OurTaggedUnion;

What's even worse is that this approach is unsafe, meaning that we can construct invalid OurTaggedUnion (i), or, for example, (ii) access data.state_1 when the actual state is OUR_TAGGED_UNION_STATE_3:

// (i)
OurTaggedUnion res1 = { .state = OUR_TAGGED_UNION_STATE_2, .data.state_1 = 123 };

// (ii)
OurTaggedUnion res2 = { .state = OUR_TAGGED_UNION_STATE_3, .data.state_3 = .99 };
some_procedure(res2.data.state_1);

poica solves these two problems by introducing [algebraic data types] (discussed in the next section). That's how it's accomplished with poica:

choice(
    OurTaggedUnion,
    variant(State1, int)
    variant(State2, const char *)
    variant(State3, double)
);

// (i) Compilation failed!
OurTaggedUnion res1 = State2(123);

OurTaggedUnion res2 = State3(.99);
some_procedure(/* Impossible to pass state_1! */);

Sum types

For example, a binary tree like this:

Can be conveniently represented as a sum type and further manipulated using pattern matching. In the code below we first construct this binary tree, and then print all its elements to stdout:

[examples/binary_tree.c]

#include <poica.h>

#include <stdio.h>

choice(
    Tree,
    variant(Empty)
    variant(Leaf, int)
    variantMany(Node,
        field(left, struct Tree *)
        field(number, int)
        field(right, struct Tree *)
    )
);

void print_tree(const Tree *tree) {
    match(*tree) {
        of(Empty) {
            return;
        }
        of(Leaf, number) {
            printf("%d\n", *number);
        }
        ofMany(Node, (left, number, right)) {
            print_tree(*left);
            printf("%d\n", *number);
            print_tree(*right);
        }
    }
}

#define TREE(tree)                obj(tree, Tree)
#define NODE(left, number, right) TREE(Node(left, number, right))
#define LEAF(number)              TREE(Leaf(number))

int main(void) {
    const Tree *tree = NODE(NODE(LEAF(81), 456, NODE(LEAF(90), 7, LEAF(111))), 57, LEAF(123));

    print_tree(tree);
}
Output
81
456
90
7
111
57
123

Product types

If we have structures in C, why do we need product types? Well, because product types provide type introspection (discussed in the next section). A product type is represented like this:

record(
    UserAccount,
    field(name, const char *)
    field(balance, double)
    field(age, unsigned char)
);

And it can be further manipulated like an ordinary structure:

UserAccount user = {"Gandalf", 14565.322, 715};
user.name = "Mithrandir";
user.age++;
user.balance *= 2;

Type introspection

Type introspection is supported in the sense that you can query the type properties of ADTs at compile-time and then handle them somehow in your hand-written macros.

Sum types

[examples/introspection/choice.c]

#include <poica.h>

#include <stdio.h>

#include <boost/preprocessor.hpp>

#define MY_CHOICE                                                           \
    Something,                                                              \
    variant(A)                                                              \
    variant(B, int)                                                         \
    variantMany(C, field(c1, double) field(c2, char))

choice(MY_CHOICE);
#define Something_INTROSPECT POICA_CHOICE_INTROSPECT(MY_CHOICE)

int main(void) {
    puts(BOOST_PP_STRINGIZE(Something_INTROSPECT));
}
Output
((POICA_VARIANT_KIND_EMPTY)(A))
((POICA_VARIANT_KIND_SINGLE)(B)(int))
((POICA_VARIANT_KIND_MANY)(C)( ((c1)(double)) ((c2)(char)) ))

Product types

[examples/introspection/record.c]

#include <poica.h>

#include <stdio.h>

#include <boost/preprocessor.hpp>

#define MY_RECORD                                                           \
    Something,                                                              \
    field(a, int)                                                           \
    field(b, const char *)                                                  \
    field(c, double)

record(MY_RECORD);
#define Something_INTROSPECT POICA_RECORD_INTROSPECT(MY_RECORD)

int main(void) {
    puts(BOOST_PP_STRINGIZE(Something_INTROSPECT));
}
Output
((a)(int)) ((b)(const char *)) ((c)(double))

Metainformation about types is actually a sequence in the terms of Boost/Preprocessor. So the BOOST_PP_SEQ_* macros can be used further, as well as Boost/VMD and the intrinsics from poica.

Safe, consistent error handling

ADTs provide a safe, consistent approach to error handling. A procedure that can fail returns a sum type, designating either a successful or a failure value, like this:

typedef enum RecvMsgErrKind {
    BAD_CONN,
    NO_SUCH_USER,
    ...
} RecvMsgErrKind;

typedef const char *Msg;

DefRes(Msg, RecvMsgErrKind);

P(Res, Msg, RecvMsgErrKind) recv_msg(...) { ... }

And then P(Res, Msg, RecvMsgErrKind) can be matched to decide what to do in the case of P(Ok, Msg, RecvMsgErrKind) and P(Err, Msg, RecvMsgErrKind):

P(Res, Msg, RecvMsgErrKind) res = recv_msg(...);
match(res) {
    of(P(Ok, Msg, RecvMsgErrKind), msg) { ... }
    of(P(Err, Msg, RecvMsgErrKind), err_kind) { ... }
}

But why this is better than int error codes? Because of:

  • Readability. Such identifiers as Ok and Err are more for humans, and therefore, it's much harder to confuse them with each other. In contrast to this, the usual approach in C to determine an error is by using magic ranges (for example, <0 or -1).

  • Consistency. No need to invent different strategies to handle different kinds of errors (i.e. using exceptions for less likely errors, int codes for a normal control flow, ...); ADTs address the problem of error handling generally.

  • Exhaustiveness checking (case analysis). A smart compiler and static analysis tools ensure that all the variants of Res are handled in match, so we can't forget to handle an error and make a possibly serious bug by leaving an application work as there's no error, when there is.

ADTs even have advantages over exceptions: they do not perform transformations with a program stack, since they are just values with no implicit logic that can hurt performance.

See examples/error_handling.c as an example of error handling using ADTs.

Built-in ADTs

ADT Description Example
Maybe An optional value examples/maybe.c
Either Either this value or that examples/either.c
Pair A pair of elements examples/pair.c
Res Either a successful or a failure value examples/error_handling.c

The last one has been presented in the previous section. All these generic types share the common API:

// Generate a definition of an ADT.
DefX(T1, ..., Tn);

// Generate a type name.
P(X, T1, ..., Tn) = ...;

The utility functions can be found in the specification.

OOP

Interfaces

[examples/interfaces.c]

#include <poica.h>

#include <math.h>
#include <stdio.h>

interface(
    Shape,
    double (*area)(const void *self);
);

record(
    Square,
    field(width, double)
    field(height, double)
);

record(
    Triangle,
    field(a, double)
    field(b, double)
    field(c, double)
);

impl(
    (Shape) for (Square),
    (double)(area)(const void *self)(
        const Square *square = (const Square *)self;
        return square->width * square->height;
    )
);

impl(
    (Shape) for (Triangle),
    (double)(area)(const void *self)(
        const Triangle *triangle = (const Triangle *)self;
        double a = triangle->a, b = triangle->b, c = triangle->c;

        double p = (a + b + c) / 2;
        return sqrt(p * (p - a) * (p - b) * (p - c));
    )
);

int main(void) {
    const Square square = { .width = 6, .height = 3.4 };
    const Triangle triangle = { .a = 4, .b = 13, .c = 15};

    printf("%f\n", iMethods(Shape, Square).area(&square));
    printf("%f\n", iMethods(Shape, Triangle).area(&triangle));
}
Output
20.400000
24.000000

Dynamic dispatch

[examples/dyn_dispatch.c]

#include <poica.h>

#include <stdio.h>

interface(
    Animal,
    void (*noise)(void *self);
);

record(Dog, field(counter, int));
record(Cat, field(counter, int));

impl(
    (Animal) for (Dog),
    (void)(noise)(void *self)(
        Dog *dog = (Dog *)self;
        dog->counter++;
        printf("Woof! Counter: %d\n", dog->counter);
    )
);

impl(
    (Animal) for (Cat),
    (void)(noise)(void *self)(
        Cat *cat = (Cat *)self;
        cat->counter++;
        printf("Meow! Counter: %d\n", cat->counter);
    )
);

int main(void) {
    Dog dog = {.counter = 0};
    Cat cat = {.counter = 0};

    AnimalMut animal;

    animal = P(newIObj, AnimalMut, Dog)(&dog);

    vCall(animal, noise);
    vCall(animal, noise);
    vCall(animal, noise);

    animal = P(newIObj, AnimalMut, Cat)(&cat);

    vCall(animal, noise);
    vCall(animal, noise);
}
Output
Woof! Counter: 1
Woof! Counter: 2
Woof! Counter: 3
Meow! Counter: 1
Meow! Counter: 2

Type-generic programming

This problem is often addressed via void * in C. However, it has two big disadvantages:

  • A compiler is unable to perform type-specific optimisations;
  • void * types could be confused with each other;
  • Not self-documenting.

poica uses a technique called monomorphisation, which means that it'll instantiate your generic types with concrete substitutions after preprocessing, eliminating all the disadvantages of void *.

Generic types

Below is a trivial implementation of a generic linked list:

[examples/generic_linked_list.c]

#include <poica.h>

#include <assert.h>
#include <stddef.h>
#include <stdlib.h>
#include <string.h>

#define DeclLinkedList(type)                                                   \
    typedef struct P(LinkedList, type) {                                       \
        type *data;                                                            \
        struct P(LinkedList, type) * next;                                     \
    } P(LinkedList, type);                                                     \
                                                                               \
    static P(LinkedList, type) * P(listNew, type)(type item);                  \
    static void P(listFree, type)(P(LinkedList, type) * list);                 \
                                                                               \
    POICA_FORCE_SEMICOLON

#define DefLinkedList(type)                                                    \
    static P(LinkedList, type) * P(listNew, type)(type item) {                 \
        P(LinkedList, type) *list = malloc(sizeof(*list));                     \
        assert(list);                                                          \
                                                                               \
        list->data = malloc(sizeof(type));                                     \
        assert(list->data);                                                    \
        memcpy(list->data, &item, sizeof(type));                               \
        list->next = NULL;                                                     \
                                                                               \
        return list;                                                           \
    }                                                                          \
                                                                               \
    static void P(listFree, type)(P(LinkedList, type) * list) {                \
        P(LinkedList, type) *node = list;                                      \
                                                                               \
        do {                                                                   \
            free(node->data);                                                  \
            P(LinkedList, type) *next_node = node->next;                       \
            free(node);                                                        \
            node = next_node;                                                  \
        } while (node);                                                        \
    }                                                                          \
                                                                               \
    POICA_FORCE_SEMICOLON

DeclLinkedList(int);
DefLinkedList(int);

int main(void) {
    P(LinkedList, int) *list = P(listNew, int)(123);
    list->next = P(listNew, int)(456);
    list->next->next = P(listNew, int)(789);

    P(listFree, int)(list);
}

There's nothing much to say, except that P (which stands for polymorphic) expands to a unique function or type identifier, e.g. performs type substitution.

Options

There are several options, implemented via macro definitions (turned off by default):

  • POICA_USE_PREFIX -- removes all the public unprefixed camelCaseed and PascalCaseed identifiers (match, DefRes, ...) from the current translation unit. The prefixed versions (poicaMatch, PoicaDefRes, ...) are defined unconditionally.
  • POICA_ENABLE_ASSERTIONS -- enables some consistency checks on input data to macros. Can increase compilation time!

FAQ

Q: What "poica" means?

A: "poica" is a Quenya word, which means clean, pure. It reflects its API.


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