struct evaluable placeholder(int dims, int *shape);
struct evaluable variable(int dims, int *shape);
struct evaluable constant(int dims, int *shape);
struct evaluable add(struct evaluable *a, struct evaluable *b);
struct evaluable mul(struct evaluable *a, struct evaluable *b);
struct evaluable dot(struct evaluable *a, struct evaluable *b, int dims_num, int *a_dims, int *b_dims);
struct evaluable sum(struct evaluable *a, int dims_num = 0, int *dims = 0);

struct evaluable sigmoid(struct evaluable *a);
struct evaluable tanh(struct evaluable *a);


enum eval_type {
    CONSTANT,
    PLACEHOLDER,
    VARIABLE,
    OPERATOR
};

struct evaluable {
    int dims;
    int *shape;
    eval_type type;
    double *value;
    void *object;
};

enum op_type {
    ELEMENT_WISE,
    UNARY,
    VARIABLE,
    OPERATOR
};

struct operator {
    struct evaluable *args;

    op_type type;
}

/*

operator
    identity
    add
    subtract
    mul(evaluable *left, evaluable *right)
    dot(evaluable *left, evaluable *right, int left_dims_size, int *left_dims, int left_dims_size, int *left_dims)

mul()


value -> double
 mapped

map

gradient

*/