maybe.h

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#ifndef IT_MAYBE_H
#define IT_MAYBE_H
 
#include <stdio.h>
#include <stdlib.h>
 
typedef enum
{
    MaybeTag_Nothing = 0, 
    MaybeTag_Just         
} MaybeTag;
 
#define Maybe(T) Maybe##T
 
#define DefineMaybe(T)                                                                                                 \
    typedef struct                                                                                                     \
    {                                                                                                                  \
        MaybeTag tag;                                                                                                  \
        /* Don't access this member manually */                                                                        \
        T val;                                                                                                         \
    } Maybe(T);                                                                                                        \
    static inline T T##_from_just(Maybe(T) maybex)                                                                     \
    {                                                                                                                  \
        if (is_just(maybex)) {                                                                                         \
            return maybex.val;                                                                                         \
        } else {                                                                                                       \
            fputs("Attempted to extract Just value from Nothing", stderr);                                             \
            abort();                                                                                                   \
        }                                                                                                              \
    }
 
#define Just(v, T) ((Maybe(T)){.tag = MaybeTag_Just, .val = (v)})
 
#define Nothing(T) ((Maybe(T)){0})
 
#define is_nothing(x) ((x).tag == MaybeTag_Nothing)
#define is_just(x) ((x).tag == MaybeTag_Just)
 
#define from_just(x, T) T##_from_just(x)
 
#define from_just_(x) (x).val
 
#endif /* !IT_MAYBE_H */