Funky C for literate programming

1 Main ideas

This is a port of LLIte in C. The reason for it is to experiment with writing functional code in standard C and compare the experience with using a functional language like F#. It is in a way a continuation of this and this posts.

I will be using glib and an header of convenient macros/functions to help me (lutils.h). I don’t think that is cheating. Any modern C praticoner has its bag of tricks …

Don’t tell me this is not idiomatic C. I already know that.

#include <string.h>
#include <stdbool.h>

#include <glib.h>
#include <glib/gprintf.h>

#ifdef ARENA
#include "arena.h"

#include "lutils.h"

2 Lack of tuples

In the snippet below I overcomed such deficiency by declaring a struct. Using the new constructor syntax makes initializing a static table simple.

typedef struct LangSymbols { char language[40]; char start[10]; char end[10];} LangSymbols;

LangSymbols* s_lang_params_table[] = {
    &(LangSymbols) {.language = "fsharp",   .start = "(*" "*", .end = "*" "*)"},
    &(LangSymbols) {.language = "c",        .start = "/*" "*", .end = "*" "*/"},
    &(LangSymbols) {.language = "csharp",   .start = "/*" "*", .end = "*" "*/"},
    &(LangSymbols) {.language = "java",     .start = "/*" "*", .end = "*" "*/"},

3 Folding over arrays

I need to gather all the languages, aka perform a fold over the array. You might have noticed the propensity to add a NULL terminator marker to arrays (as for strings). This allows me to avoid passing a size to functions and makes simpler writing utility macros (as foreach below) more simply.

In the rest of the program, every time I end a function with _z, it is because I consider it generally usable and I add a version of it without the _z to lutils.h.

#define array_foreach_z(p) for(; *symbols != NULL; ++symbols)

char* summary(LangSymbols** symbols) {

    GString* langs = g_string_sized_new(20);
    array_foreach(symbols) g_string_append_printf(langs, "%s ", (*symbols)->language);

    g_string_truncate(langs, strlen(langs->str) - 1);

    GString* usage = g_string_sized_new(100);

        "You should specify:nt. either -l or -o and -pn"
        "t. either -indent or -P and -Cn"
        "t. -l supports: %s"

    return usage->str;

Find an item in an array based on some expression. Returns NULL if not found. Again, this is a common task, hence I’ll abstract it out with a macro (that ends up being a cute use of gcc statment expressions).

#define array_find_z(arr, ...)                          
        array_foreach(arr) if (__VA_ARGS__) break;      

LangSymbols* lang_find_symbols(LangSymbols** symbols, char* lang) {

    return array_find(symbols, !strcmp((*symbols)->language, lang));

4 Deallocating stuff

You might wonder why I don’t seem overly worried about deallocating the memory that I allocate. I haven’t gone crazy(yet). You’ll see.

5 Discriminated unions

Here are the discriminated unions macros from a previous blog post of mine. I’ll need a couple of these and pre-declare two functions.

union_decl(CodeSymbols, Indented, Surrounded)
    union_type(Indented,    int indentation;)
    union_type(Surrounded,  char* start_code; char* end_code;)

typedef struct Options {
    char*           start_narrative;
    char*           end_narrative;
    CodeSymbols*    code_symbols;
} Options;

gchar* translate(Options*, gchar*);

union_decl(Block, Code, Narrative)
    union_type(Code,        char* code)
    union_type(Narrative,   char* narrative)

6 Main data structure

We want to use higher level abstractions that standard C arrays, hence we’ll pick a convenient data structure to use in the rest of the code. A queue lets you to insert at the front and back, with just a one pointer overhead over a single linked list. Hence it is my data structure of choice for this program.

GQueue* blockize(Options*, char*);

There is already a function in glib to check if a string has a certain prefix (g_str_has_prefix). We need one that returns the remaining string after the prefix. We also define a g_slow_assert that is executed just if G_ENABLE_SLOW_ASSERT is defined

char* str_after_prefix(char* src, char* prefix) {
    g_slow_assert(g_str_has_prefix(src, prefix));

    while(*prefix != '0')
        if(*src == *prefix) ++src, ++prefix;
        else break;

    return src;

7 Tokenizer

The structure of the function is identical to the F# version. The big bread-winners are statement expressions and local functions …

It is interesting how you can replicate the shape of an F# function by substituting ternary operators for match statements.

It is nothing magic, just a way to have a case statment as an expression, but it is suggestive of its more functional counterpart.

#define NL "n"

union_decl(Token, OpenComment, CloseComment, Text)
    union_type(OpenComment, int line)
    union_type(CloseComment,int line)
    union_type(Text,        char* text)

GQueue* tokenize(Options* options, char* source) {

    struct tuple { int line; GString* acc; char* rem;};

    bool is_opening(char* src)      { return g_str_has_prefix(src, options->start_narrative);}
    bool is_closing(char* src)      { return g_str_has_prefix(src, options->end_narrative);}
    char* remaining_open (char* src){ return str_after_prefix(src, options->start_narrative);}
    char* remaining_close(char* src){ return str_after_prefix(src, options->end_narrative);}

    struct tuple text(char* src, GString* acc, int line) {
        inline struct tuple stop_parse_text()
            { return (struct tuple) {.line = line, .acc = acc, .rem = src};}

        return  str_empty (src)? stop_parse_text() :
                is_opening(src)? stop_parse_text() :
                is_closing(src)? stop_parse_text() :
                                  int line2         = g_str_has_prefix(src, NL) ? line + 1
                                                                                : line;
                                  GString* newAcc   = g_string_append_c(acc, *src);
                                  char* rem         = src + 1;
                                  text(rem, newAcc, line2);

    GQueue* tokenize_rec(char* src, GQueue* acc, int line) {
        return  str_empty(src)  ?   acc                     :
                is_opening(src) ?   tokenize_rec(remaining_open(src),
                                        g_queue_push_back(acc, union_new(
                                                    Token, OpenComment, .line = line)),
                                        line)        :
                is_closing(src) ?   tokenize_rec(remaining_close(src),
                                               g_queue_push_back(acc, union_new(
                                                    Token, CloseComment, .line = line)),
                                        line)        :
                                    struct tuple t = text(src, g_string_sized_new(200), line);
                                        g_queue_push_back(acc, union_new(
                                                    Token, Text, .text = t.acc->str)), t.line);

    return tokenize_rec(source, g_queue_new(), 1);

8 Parser

This again has a similar structure to the F# version, just longer. It is very long because it contains 3 (nested) functions which are on the verbose side in C.

The creation of a error macro is unfortunate. I just don’t know how to adapt g_assert_e so that it works for not pointer returning functions.

I also need a simple function report_error to exit gracefully giving a message to the user. I didn’t found such thing in glib (?)

#define report_error_z(...) G_STMT_START { g_print(__VA_ARGS__); exit(1); } G_STMT_END                                                            

union_decl(Chunk, NarrativeChunk, CodeChunk)
    union_type(NarrativeChunk,  GQueue* tokens)
    union_type(CodeChunk,       GQueue* tokens)

GQueue* parse(Options* options, GQueue* tokens) {

    struct tuple { GQueue* acc; GQueue* rem;};

    #define error(...) 
        ({ report_error(__VA_ARGS__); (struct tuple) {.acc = NULL, .rem = NULL}; })

    struct tuple parse_narrative(GQueue* acc, GQueue* rem) {

        bool isEmpty    = g_queue_is_empty(rem);
        Token* h        = g_queue_pop_head(rem);
        GQueue* t       = rem;

        return  isEmpty                 ?
                                    error("You haven't closed your last narrative comment") :
                h->kind == OpenComment  ?
                    error("Don't open narrative comments inside narrative comments at line %i",
                          h->OpenComment.line)                                              :
                h->kind == CloseComment ? (struct tuple) {.acc = acc, .rem = t}             :
                h->kind == Text         ? parse_narrative(g_queue_push_back(acc, h), t)     :
                                          error("Should never get here");

    struct tuple parse_code(GQueue* acc, GQueue* rem) {

        bool isEmpty    = g_queue_is_empty(rem);
        Token* h    = g_queue_pop_head(rem);
        GQueue* t   = rem;

        return  isEmpty                 ? (struct tuple) {.acc = acc, .rem = t}         :
                h->kind == OpenComment  ?
                    (struct tuple) {.acc = acc, .rem = g_queue_push_front(rem, h)}      :
                h->kind == CloseComment ? parse_code(g_queue_push_back(acc, h), rem)    :
                h->kind == Text         ? parse_code(g_queue_push_back(acc, h), rem)    :
                                          error("Should never get here");
    #undef error

    GQueue* parse_rec(GQueue* acc, GQueue* rem) {

        bool isEmpty    = g_queue_is_empty(rem);
        Token* h    = g_queue_pop_head(rem);
        GQueue* t   = rem;

        return  isEmpty                 ? acc                                           :
                h->kind == OpenComment  ? ({
                                           GQueue* emp = g_queue_new();
                                           struct tuple tu = parse_narrative(emp, t);
                                           Chunk* ch = union_new(
                                                Chunk, NarrativeChunk, .tokens = tu.acc );
                                           GQueue* newQ = g_queue_push_back(acc, ch);
                                           parse_rec(newQ, tu.rem);
                                           })                                            :
                h->kind == CloseComment ?
                        "Don't insert a close narrative comment at the start of your"
                        " program at line %i",
                                            h->OpenComment.line)                         :
                h->kind == Text         ?
                                           GQueue* emp = g_queue_new();
                                           struct tuple tu =
                                                parse_code(g_queue_push_front(emp, h), t);
                                             union_new(Chunk, CodeChunk, .tokens = tu.acc)),
                                          })                                                               :

    return parse_rec(g_queue_new(), tokens);

9 Flattener

This follows the usual practice of representing fold as foreach statments (and maps to). Pheraps I shall build better abstractions for them at some point. I also introduce a little macro to simplify writing of GFunc lambdas, given how pervasive they are.

Again, note how heavy ternary operated this is …

#define g_func_z(type, name, ...) lambda(void,                                              
                                        (void* private_it, G_GNUC_UNUSED void* private_no){ 
                                       type name = private_it;                              

GQueue* flatten(Options* options, GQueue* chunks) {
    GString* token_to_string_narrative(Token* tok) {
        return  tok->kind == OpenComment ||
                tok->kind == CloseComment   ?
                    report_error_e("Cannot nest narrative comments at line %i",
                                   tok->OpenComment.line)                                   :
                tok->kind == Text           ? g_string_new(tok->Text.text)                  :
    GString* token_to_string_code(Token* tok) {
        return  tok->kind == OpenComment    ?
                    "Open narrative comment cannot be in code at line %i."
                    " Pheraps you have an open comment "
                    "in a code string before this comment tag?"
                    , tok->OpenComment.line)                                                :
                tok->kind == CloseComment   ? g_string_new(options->end_narrative)          :
                tok->kind == Text           ? g_string_new(tok->Text.text)                  :
    Block* flatten_chunk(Chunk* ch) {
        return  ch->kind == NarrativeChunk  ? ({
                               GQueue* tokens = ch->NarrativeChunk.tokens;
                               GString* res = g_string_sized_new(256);
                               g_queue_foreach(tokens, g_func(Token*, tok,
                                                ), NULL);
                               union_new(Block, Narrative, .narrative = res->str);
                                               })   :
                ch->kind == CodeChunk       ? ({
                               GQueue* tokens = ch->CodeChunk.tokens;
                               GString* res = g_string_sized_new(256);
                               g_queue_foreach(tokens, g_func(Token*, tok,
                                                        ), NULL);
                               union_new(Block, Code, .code = res->str);
                                               })   :

    GQueue* res = g_queue_new();
    g_queue_foreach(chunks, g_func(Chunk*, ch,
                                Block* b = flatten_chunk(ch);
                                g_queue_push_tail(res, b);
                                ) ,NULL);
    return res;

Now we can tie everything together to build blockize, which is our parse tree.

GQueue* blockize(Options* options, char* source) {
    GQueue* tokens  = tokenize(options, source);
    GQueue* blocks  = parse(options, tokens);
    return flatten(options, blocks);

10 Define the phases

In C you can easily forward declare function, so you don’t have to come up with some clever escabotage like we had to do in F#.

GQueue* remove_empty_blocks(Options*, GQueue*);
GQueue* merge_blocks(Options*, GQueue*);
GQueue* add_code_tags(Options*, GQueue*);

GQueue* process_phases(Options* options, GQueue* blocks) {

    blocks          = remove_empty_blocks(options, blocks);
    blocks          = merge_blocks(options, blocks);
    blocks          = add_code_tags(options, blocks);
    return blocks;

char* extract(Block* b) {
    return  b->kind == Code         ? b->Code.code          :
            b->kind == Narrative    ? b->Narrative.narrative:

There must be a higher level way to write this utility function …

bool is_str_all_spaces(const char* str) {
    while(*str != '0') {
            return false;
    return true;

GQueue* remove_empty_blocks(G_GNUC_UNUSED Options* options, GQueue* blocks) {

    g_queue_foreach(blocks, g_func(Block*, b,
            g_queue_remove(blocks, b);
                                   ), NULL);
    return blocks;

GQueue* merge_blocks(G_GNUC_UNUSED Options*options, GQueue* blocks) {
    return  g_queue_is_empty(blocks)            ? blocks            :
            g_queue_get_length(blocks) == 1     ? blocks            :
                 Block* h1 = g_queue_pop_head(blocks);
                 Block* h2 = g_queue_pop_head(blocks);
                 h1->kind == Code && h2->kind == Code ? ({
                     char* newCode =
                        g_strjoin("", h1->Code.code, NL, h2->Code.code, NULL);
                     Block* b = union_new(Block, Code, .code = newCode);
                     merge_blocks(options, g_queue_push_front(blocks, b));
                                                         })         :
                 h1->kind == Narrative && h2->kind == Narrative ? ({
                     char* newNarr =
                            "", h1->Narrative.narrative, NL, h2->Narrative.narrative, NULL);
                     Block* b = union_new(Block, Narrative, .narrative = newNarr);
                     merge_blocks(options, g_queue_push_front(blocks, b));
                                                         })         :
                     GQueue* newBlocks =
                        merge_blocks(options, g_queue_push_front(blocks, h2));
                     g_queue_push_front(newBlocks, h1);

This really should be in glib …

inline static
gint g_asprintf_z(gchar** string, gchar const *format, ...) {
	va_list argp;
	va_start(argp, format);
	gint bytes = g_vasprintf(string, format, argp);
    return bytes;

char* indent(int n, char* s) {

    char* ind       = g_strnfill(n, ' ');
    char* tmp;
    g_asprintf(&tmp, "%s%s", ind, s);

    char* withNl;
    g_asprintf(&withNl, "n%s", ind);

    return g_strjoinv(withNl, g_strsplit(tmp, NL, -1));

And finally I ended up defining map. See if you like how the usage looks in the function below.

#define g_queue_map_z(q, type, name, ...) ({                                
        GQueue* private_res = g_queue_new();                                
        g_queue_foreach(q, g_func(type, name,                               
            name = __VA_ARGS__;                                             
            g_queue_push_tail(private_res, name);                           
            ), NULL);                                                       

GQueue* add_code_tags(Options* options, GQueue* blocks) {

    GQueue* indent_blocks(GQueue* blocks) {
        return g_queue_map(blocks, Block*, b,
                b->kind == Narrative ? b                                                                                                    :
                b->kind == Code      ?
                    union_new(Block, Code, .code =
                        indent(options->code_symbols->Indented.indentation, b->Code.code))    :

    GQueue* surround_blocks(GQueue* blocks) {
        return g_queue_map(blocks, Block*, b,
                b->kind == Narrative ?
                    union_new(Block, Narrative, .narrative =
                        g_strjoin("", NL, g_strstrip(b->Narrative.narrative), NL, NULL))   :
                b->kind == Code      ?
                    union_new(Block, Code, .code = g_strjoin("",
                                                 NULL))    :


    return  options->code_symbols->kind == Indented     ?   indent_blocks(blocks)   :
            options->code_symbols->kind == Surrounded   ?   surround_blocks(blocks) :

char* stringify(GQueue* blocks) {
    GString* res = g_string_sized_new(2048);
    g_queue_foreach(blocks, g_func(Block*, b,
        g_string_append(res, extract(b));
    ), NULL);
    return g_strchug(res->str);

void deb(GQueue* q);

char* translate(Options* options, char* source) {

    GQueue* blocks  = blockize(options, source);
    blocks          = process_phases(options, blocks);
    return stringify(blocks);

11 Parsing the command line

In glib there is a command line parser that accept options in unix-like format and automatically produces professional --help messages and such. We shoudl really have something like this in .NET. Pheraps we do and I’m not aware of it?

typedef struct CmdOptions { char* input_file; char* output_file; Options* options;} CmdOptions;

CmdOptions* parse_command_line(int argc, char* argv[]);

static char *no = NULL, *nc = NULL, *l = NULL, *co = NULL, *cc = NULL, *ou = NULL;
static char** in_file;

static int ind = 0;
static bool tests = false;

// this is a bug in gcc, fixed in 2.7.0 not to moan about the final NULL
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wmissing-field-initializers"

static GOptionEntry entries[] =
  { "language"          , 'l', 0, G_OPTION_ARG_STRING, &l ,
                                "Language used", "L"  },
  { "output"            , 'o', 0, G_OPTION_ARG_FILENAME, &ou,
                                "Defaults to the input file name with mkd extension", "FILE" },
  { "narrative-open"    , 'p', 0, G_OPTION_ARG_STRING, &no,
                                "String opening a narrative comment",   "NO" },
  { "narrative-close"   , 'c', 0, G_OPTION_ARG_STRING, &nc,
                                "String closing a narrative comment",   "NC" },
  { "code-open"         , 'P', 0, G_OPTION_ARG_STRING, &co,
                                "String opening a code block",          "CO" },
  { "code-close"        , 'C', 0, G_OPTION_ARG_STRING, &cc,
                                "String closing a code block",          "CC" },
  { "indent"            , 'i', 0, G_OPTION_ARG_INT,    &ind,
                                "Indent the code by N whitespaces",    "N"  },
  { "run-tests"         , 't', G_OPTION_FLAG_HIDDEN, G_OPTION_ARG_NONE,   &tests,
                                "Run all the testcases", NULL },
                                "Input file to process",   "FILE" },
  { NULL }
#pragma GCC diagnostic pop

Brain damaged way to run tests with a -t hidden option. Not paying the code size price in release.

#ifndef NDEBUG
#include "tests.c"

Here is my big ass command parsing function. It could use a bit of refactoring …

void destroy_arena_allocator();

CmdOptions* parse_command_line(int argc, char* argv[]) {

    GError *error = NULL;
    GOptionContext *context;

    context =
        g_option_context_new ("- translate source code with comemnts to an annotated file");
    g_option_context_add_main_entries (context, entries, NULL);
    g_option_context_set_summary(context, summary(s_lang_params_table));

    if (!g_option_context_parse (context, &argc, &argv, &error))
        report_error("option parsing failed: %s", error->message);

    CmdOptions* opt = g_new(CmdOptions, 1);
    opt->options = g_new(Options, 1);

    #ifndef NDEBUG
    if(tests) {
        int i = run_tests(argc, argv);

    if(!in_file) report_error("No input file");
    opt->input_file = *in_file;

    // Uses input file without extension, adding extension .mkd (assume markdown)
    opt->output_file = ou ? ou :  ({
                                  char* output      = g_strdup(*in_file);
                                  char* extension   = g_strrstr(output, ".");
                                  extension ? ({
                                               *extension = '0';
                                               g_strjoin("", output, ".mkd", NULL);
                                                }) :
                                               g_strjoin("", output, ".mkd", NULL);

    if(l) { // user passed a language
        LangSymbols* lang = lang_find_symbols(s_lang_params_table, l);
        if(!lang) report_error("%s is not a supported language", l);

        opt->options->start_narrative  = lang->start;
        opt->options->end_narrative    = lang->end;

    } else {
        if(!no || !nc) report_error("You need to specify either -l, or both -p and -c");

        opt->options->start_narrative  = no;
        opt->options->end_narrative    = nc;

    if(ind) { // user pass    g_option_context_free();
        opt->options->code_symbols = union_new(CodeSymbols, Indented, .indentation = ind);
    } else {
        if(!co || !cc) report_error("You need to specify either -indent, or both -P and -C");
        opt->options->code_symbols =
            union_new(CodeSymbols, Surrounded, .start_code = co, .end_code = cc);

    return opt;

Some windows programs (i.e. notepad, VS, …) add a 3 bytes prelude to their utf-8 files, C doesn’t know anything about it, so you need to strip it. On this topic, I suspect the program works on UTF-8 files that contain non-ASCII chars, even if when I wrote it I didn’t know anything about localization.

It should work because I’m just splitting the file when I see a certain ASCII string and in UTF-8 ASCII chars cannot appear anywhere else than in their ASCII position.

char* skip_utf8_bom(char* str) {
    unsigned char* b = (unsigned char*) str;
    return  b[0] == 0xEF && b[1] == 0xBB && b[2] == 0xBF    ? (char*) &b[3]  : // UTF-8
                                                              (char*) b;

12 Not freeing memory (again)

The reason I haven’t been freeing memory all along is because I was planning on using an arena allocator (a kind of linear allocator).

Memory management is fully hortogonal to the style of programming described in this post. You can do it whatever way you prefer, but there is a certain affinity between an arena allocator (or garbage collection) and functional programming because of the temporary objects created in expressions. You could create the temporary objects explicitely, but that would diminish the conciseness of the paradigm.

I have an arena allocator implementation here. In the code below I comment it out so that you don’t have a dependency from that code if you want to try this. The program runs so quickly and it does so little that you can probably let the operating system reclame memory at the end of the process life.

If you ended up integrating this with an editor (i.e. literate programming editing), you’d need to be more careful.

#ifdef ARENA

Arena_T the_arena;

inline static
gpointer arena_malloc(gsize n_bytes) {
    return Arena_alloc(the_arena, n_bytes, __FILE__, __LINE__);

inline static
gpointer arena_calloc(gsize n_blocks, gsize n_block_bytes) {
    return Arena_calloc(the_arena, n_blocks, n_block_bytes, __FILE__, __LINE__);

inline static
gpointer arena_realloc(gpointer mem, gsize n_bytes) {
    return Arena_realloc(the_arena, mem, n_bytes, __FILE__, __LINE__);

void arena_free(G_GNUC_UNUSED gpointer mem) {
    // NOP

void set_arena_allocator() {
    GMemVTable vt = (GMemVTable) { .malloc = arena_malloc,      .calloc = arena_calloc,
                                   .realloc = arena_realloc,    .free = arena_free,
                                   .try_malloc = arena_malloc,  .try_realloc = arena_realloc};

    the_arena = Arena_new();

void destroy_arena_allocator() {


13 Summary

I have to say, it didn’t feel too cumbersome to structure C code in a functional way, assuming that you can use GLib and a couple of GCC extensions to the language. It certainly doesn’t have the problems that C++ has in terms of debugging STL failures.

There are a couple of things I don’t like about GLib and I’m working on an hobby project to overcome them. Eventually I’ll post it.

int main(int argc, char* argv[])
#ifdef ARENA

    CmdOptions* opt = parse_command_line(argc, argv);

    char* source    = NULL;
    GError* error   = NULL;

    if(!g_file_get_contents(opt->input_file, &source, NULL, &error))

    source = skip_utf8_bom(source);

    char* text              = translate(opt->options, source);

    if(!g_file_set_contents(opt->output_file, text, -1, &error))

#ifdef ARENA

    return 0;

Functional programming in C – Implementation

0.1 Cleanup

Let’s start simple with the cleanup function. First we need the usual barrage of includes. G_BEGIN_DECLS allows the header to be linked in C++.


#include "glib.h"


#include <stdlib.h>
#include <stdio.h>

This feature is GCC specific. It uses __attribute((cleanup(f))) where f is the cleanup function. In this case the cleanup function just frees the memory.

#ifdef __GNUC__

 static inline void __autofree(void *p) {
     void **_p = (void**)p;

auto_clean is a building block that you can use to plug in your own cleanup function. In the common case of memory allocation, I created a wrapper macro auto_free to make it even easier.

#define auto_clean(f)   __attribute((cleanup(f)))
#define auto_free       auto_clean(__autofree)

0.2 Lambdas

I took this one from here.

If you think about it, a lambda is just an expression that returns a function. This macro creates a nested function, called fn, inside a statement expression and returns it. Unfortunately these features are gcc specific.

Remember that lambdas are not allocated on the heap, so you have to be careful on how you used them.

#define lambda(return_type, function_body)                                          
    return_type __fn__ function_body                                                


0.3 Unions

A union type is what you would expect: a struct that contains an unnamed union and a field to specify which type it is. We need the list of types in union_decl to create the kind enum. The usage of __VA_ARGS__ allows to use whatever syntax you want to go into the enum (i.e. specify int values).

Having to specify the the types here is unfortunate as you are going to need to specify it in the union_case macros as well.

I haven’t found another way to do it. If you do, let me know.

#define union_decl(alg, ...)                                                        
typedef struct alg {                                                                
    enum {  __VA_ARGS__ } kind;                                                     
    union {

You specify each type for the union with union_type. That looks pretty good to me.

#define union_type(type, ...)                                                       
    struct type { __VA_ARGS__ } type;

Ideally you shouldn’t need to specify alg here. Perhaps there is a way to avoid doing so.

#define union_end(alg)                                                              
    };} alg;

You can then set the fields on the union type by using the below macro. Notice the usage of the new struct constructor here to allow optional named parameters.

This is a statement, so it cannot go into an expression place. I think I could make it an expression that returns the existing (or a new) union. This is going to be a sceanrio if people are not using gcc statement expressions.

#define union_set(instance, type, ...)                                              
    G_STMT_START {                                                                  
        (instance)->kind     = (type);                                              
        (instance)->type   = (struct type) { __VA_ARGS__ };                         
    } G_STMT_END

This is an utility macro. It is a version of g_assert that you can use in an expression position.

#define g_assert_e(expr) (                                                          
    (G_LIKELY (!expr) ?                                                             
   (void)g_assertion_message_expr (G_LOG_DOMAIN, __FILE__, __LINE__, G_STRFUNC, 
    : (void) 1) )

And this allows to fill the default case in a match statement implemented as a ternary operator. It prints out a text representation of the expression and returns it.

#define union_fail(...) (g_assert_e(((void)(__VA_ARGS__) , false)), (__VA_ARGS__))

The rest of the code is commented out. It is a macro way to do pattern matching. For me, the ternary operator is simpler, but I left it there in case you want to play with it.

#define union_case_only_s(instance, type, ...)                                      
        G_STMT_START {                                                              
        if((instance)->kind == (type)) {                                            
            G_GNUC_UNUSED struct type* it = &((instance)->type); __VA_ARGS__; }     
        else g_assert_not_reached();                                                
        } G_STMT_END

#define union_case_first_s(alg, instance, type, ...)                                
    G_STMT_START {                                                                  
        alg* private_tmp = (instance);                                              
        if(private_tmp->kind == type) {                                             
            G_GNUC_UNUSED struct type* it = &((private_tmp)->type); __VA_ARGS__; }

#define union_case_s(type, ...)                                                     
        else if(private_tmp->kind == type) {                                        
            G_GNUC_UNUSED struct type* it = &((private_tmp)->type); __VA_ARGS__; }

#define union_case_last_s(type, ...)                                                
        else if(private_tmp->kind == type) {                                        
            G_GNUC_UNUSED struct type* it = &((private_tmp)->type); __VA_ARGS__; }  
            else g_assert_not_reached(); } G_STMT_END

#define union_case_default_s(...)                                                   
        else __VA_ARGS__; } G_STMT_END

// Need to use assert here because g_assert* cannot be used in expressions as it expands to do .. while(0)
#define union_case_only(instance, type, ...)                                        
        ( (instance)->kind == (type) ? (__VA_ARGS__) : (assert(false), __VA_ARGS__) )

#define union_case_first(instance, type, ...)                                       
        ( (instance)->kind == (type) ? (__VA_ARGS__) :

#define union_case(instance, type, ...)                                             
        (instance)->kind == (type) ? (__VA_ARGS__) :

#define union_case_last(instance, type, ...)                                        
        (instance)->kind == (type) ? (__VA_ARGS__) : (assert(false), (__VA_ARGS__)) )
#define union_case_default(...)                                                     
        (__VA_ARGS__) )




Functional programming in C

This post/program (as I’m writing it in literate style) is a continuation of my previous posts about functional programming in C++. I promise I’m not going to post about doing it in assembly language (I think) ….

I came to like the simplicity of C very much and got interested in how you could write functional code in it.

There is one irritating thing about C as a viable programming language. Microsoft’s compiler support is not good. It just supports ANSI C, not C99 or C11. So, if you want to use more modern idyoms, you got to use gcc or clang. In this post I assume you use gcc. I will point out the gcc specific extensions.

Also, the C standard library is pretty limited, so I decided to use GLib to complement it. I also created some macros to simplify the syntax. I never understood why people like templates and think macros are evil. It takes me all of 5 minutes to do a -E on GCC to debug the result of a macro expansion. With templates, well, that’s different.

So, in summary, this post is about how you can write functional code in C, perhaps with some gcc extensions and certainly with some macro tricks. Let’s call it funkyC (thanks Ian ). I’m going to show how to use it first. Next post I’m going to show how it’s implemented.

0.1 Discriminated unions in C

With a bit of macro magic, you can get a decent looking discriminated union syntax. First we need to include the correct headers. lutils.h is where all the macros are defined.

#include <glib.h>
#include <stdio.h>
#include <assert.h>
#include <stdbool.h>
#include <signal.h>
#include <string.h>

#include "lutils.h"

Then you can declare a discriminated union with the syntax below. It suffers from two problems: repeting the list of possible types in union_decl and repeating the name of the discriminated union in union_end. Perhaps there is a way to avoid that, but I haven’t found it.

The syntax for the union_type call is the same as you would use inside a struct declaration. We’ll see how this works when we look at lutils.h.

union_decl  (Car, Volvo, Fiat, Ferrari)
union_type      (Volvo,     int x; double y;)
union_type      (Fiat,      char* brand, *model;)
union_type      (Ferrari,   char* brand, *model;)
union_end   (Car)

We can create a Car either on the stack, as below, or on the heap and we can set its value with union_set.

Notice the usage of the new struct construction syntax to simulate optional named parameters in C. I would prefer not to have a dot at the start of the name, but overall it is beautiful (if I can say that myself).

static void printCar(Car*);

static void testUnion() {
    Car c;

    union_set   (&c, Volvo, .x = 3, .y = 4);
    printCar    (&c);
    union_set   (&c, Ferrari, .brand = "Ferrari");
    printCar    (&c);
    union_set   (&c, Fiat, .brand = "Fiat", .model = "234");
    printCar    (&c);

You can then access values inside your discriminated union with normal if statements.

static void testCar(Car*, char const *);

static void printCar(Car* c) {

    if(c->kind == Volvo) {
        int x = c->Volvo.x;
        g_assert_cmpint(x, ==, 3);

Or perhaps you want the equivalent of a match statement in F# (aka an expression that returns a value based on the type of the discriminated union). Notice that, as logical, all the expressions need to return the same type. That’s why union_fail takes a value of the expression type.

    char temp[40];

    char* value =   c->kind == Volvo    ?   itoa(c->Volvo.x, temp, 10)
                  : c->kind == Ferrari  ?   (void)c->Ferrari.model, c->Ferrari.brand
                  : c->kind == Fiat     ?   c->Fiat.model
                                        :   union_fail("Not a valid car type");

If you are willing to be gcc specific, then your expression can be comprised of multiple statements, of which the last one returns the value of the expression. This allows a much more flexible syntax for your match clauses.

#ifdef __GNUC__

    value       =   c->kind == Volvo    ? ({
                                            struct Volvo* it = &c->Volvo;
                                            itoa(it->x, temp, 10);
                  : c->kind == Ferrari  ?   (void)c->Ferrari.model, c->Ferrari.brand
                  : c->kind == Fiat     ?   c->Fiat.model
                                        :   union_fail("Not a valid car type");

    testCar(c, value);

#endif // __GNUC__

We then use the super simple test framework in GLib to make sure that it all works as expected …

static void testCar(Car* c, char const * value) {
    if(c->kind == Volvo) g_assert_cmpstr(value, ==, "3");
    else if (c->kind == Fiat) g_assert_cmpstr(value, ==, "234");
    else if (c->kind == Ferrari) g_assert_cmpstr(value, ==, "Ferrari");
    else g_assert_not_reached();


0.2 Nested functions and lambda variables

GCC has many other cool extensions. A very simple one is nested functions. It allows you to nest functions :-) Look at the definition of doA and f2 in the function below. Putting together nested functions and block statement expressions allows you, with some macro magic, to define lambda functions in your code (from here ).

Remember that lambdas (aka nested functions) are allocated on the stack. They are very fast, but you cannot store their pointer into a gloal table (unless such table is used while the stack for this function is alive).

In such cases, you have to create a proper function. But for the other 90% of use cases, they work pretty well. They are lambdas in the spirit of C: very fast, but error prone …

#ifdef __GNUC__

static void testLambda() {

    typedef int (*aFunc) (int);

    aFunc doA(aFunc f){

        int k(int i) {
            return f(i) + 3;
        return k;

    int clos = 2;

    int f2 (int i) {return i;}
    aFunc b = doA(lambda (int, (int p) {return p + clos;}));

    g_assert_cmpint(b(3), ==, 8);

0.3 Automatic cleanup of local variables

This is not a functional topic per se, but something that always annoyed me tremendously about C. The fact that you cannot define the equivalent of the using statement in C#, or destructors in C++. Well, now you can. Or not?

Again, if you are willing to be GCC specific, you can use an attribute (more on this in the upcoming implementation post) to associate a cleanup function that gets called when your variable goes out of scope. In this case, I wrapped the free case in a nice looking macro.

But that doesn’t really work. You would certainly want such function to be called on any kind of exit from the enclosing scope (aka via exit(), abort() or longjmp()). Alas, that doesn’t happen.

This reduces the usefulness of this mechanism tremendously. Probably too much in that it lulls you into a false sense of security. You still need to free your resources in the error path of your application.

static void testAutomaticCleanup() {
    char* stack_alloc() {
        auto_free char* b = g_malloc(10000);
        memset(b, '#', 10000);
        return b;

    char * c = stack_alloc();
    g_assert(*c != '#');


0.4 Data structures

GLib comes with a vast library of data structures to use, not too different from the .NET framework or Java. For example, below you have a single linked list …

static void testGLib() {
     GSList* list = NULL;

     list = g_slist_append(list, "Three");
     list = g_slist_prepend(list, "first");
     g_assert_cmpint(g_slist_length(list), ==, 2);

     list = g_slist_remove(list, "first");
     g_assert_cmpint(g_slist_length(list), ==, 1);


0.5 Wrapping up

There you go, rising the level of abstraction of C, still keeping it very fast (if you are willing to be gcc bound).

There are other features in functional programming languages that are not in this post. Maybe I’ll get around to macro my way into them eventually, maybe not.

In the next post we’ll talk about how all of this is implemented. Below is the code for running the testcases.

int runTests(int argc, char* argv[]) {
    g_test_init(&argc, &argv, NULL);

    if(g_test_quick()) {
        g_test_add_func("/utils/automatic_cleanup", testAutomaticCleanup);
        g_test_add_func("/utils/lambda", testLambda);
        g_test_add_func("/utils/Union", testUnion);
        g_test_add_func("/utils/SList", testGLib);

    return g_test_run();

int main(int argc, char *argv[]) {
    return runTests(argc, argv);