Add name-constrained.c

Author: TotallyNotChase

name-constrained.c

@@ -0,0 +1,86 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+/* The `Show` typeclass allows types to be turned into their string representation */
+typedef struct
+{
+    char* (*const show)(void* self);
+} ShowTC;
+
+typedef struct
+{
+    void* self;
+    ShowTC const* tc;
+} Show;
+
+#define CONCAT_(x, y) x ## y
+#define CONCAT(x, y) CONCAT_(x, y)
+
+/* Consistently name the impl functions */
+#define ImplName(T, TypeclassName) CONCAT(CONCAT(T, _to_), TypeclassName)
+
+/* "Apply" a typeclass over a concrete type */
+#define ap(x, T, TypeclassName) ImplName(T, TypeclassName)(x)
+
+#define impl_show(T, show_f)                                                                                           \
+    Show ImplName(T, Show)(T* x)                                                                                       \
+    {                                                                                                                  \
+        char* (*const show_)(T* self) = (show_f);                                                                      \
+        (void)show_;                                                                                                   \
+        static ShowTC const tc = {.show = (char* (*const)(void*))(show_f) };                                           \
+        return (Show){.tc = &tc, .self = x};                                                                           \
+    }
+
+/* Polymorphic printing function */
+void print(Show showable)
+{
+    char* s = showable.tc->show(showable.self);
+    puts(s);
+    free(s);
+}
+
+
+/* A very holy enum */
+typedef enum
+{
+    holy,
+    hand,
+    grenade
+} Antioch;
+
+static inline char* strdup_(char const* x)
+{
+    char* s = malloc((strlen(x) + 1) * sizeof(*s));
+    strcpy(s, x);
+    return s;
+}
+
+/* The `show` function implementation for `Antioch*` */
+static char* antioch_show(Antioch* x)
+{
+    /*
+    Note: The `show` function of a `Show` typeclass is expected to return a malloc'ed value
+    The users of a generic `Show` are expected to `free` the returned pointer from the function `show`.
+    */
+    switch (*x)
+    {
+        case holy:
+            return strdup_("holy");
+        case hand:
+            return strdup_("hand");
+        case grenade:
+            return strdup_("grenade");
+        default:
+            return strdup_("breakfast cereal");
+    }
+}
+
+/* Make function to build a generic `Show` out of a concrete type- `Antioch` */
+impl_show(Antioch, antioch_show)
+
+int main(void)
+{
+    print(ap(&(Antioch){ holy }, Antioch, Show));
+    return 0;
+}