set() function + userdata storage + Expr instance access for functions

- functions can take string arguments (i.e. variable names that may be
  undefined)

- set(<varname>, <value>) sets a variable to a user-defined value

- any userdata can be stored via expr_set_userdata() and retrieved via
  expr_get_userdata(); also accessible to functions

- functions gain access to the expr instance and consequently all
  of its variables including userdata

expr.h

@@ -5,16 +5,29 @@
 #include <stddef.h>
 #include <stdint.h>
 
+typedef struct _Expr Expr;
+
 typedef struct {
 	size_t start, end;
 	const char *err;
 } ExprError;
 
+typedef enum {
+	ExprArgTypeStr,
+	ExprArgTypeNum,
+} ExprArgType;
+
+typedef union {
+	char *Str;
+	double Num;
+} ExprArg;
+
 typedef struct {
 	const char *name;
 	const char *description;
-	double (*func)(double *args);
+	double (*func)(Expr *e, ExprArg *args);
 	const char **arg_names;
+	ExprArgType *arg_types;
 	size_t n_args;
 } ExprBuiltinFunc;
 
@@ -24,8 +37,6 @@ typedef struct {
 	double val;
 } ExprBuiltinVar;
 
-typedef struct _Expr Expr;
-
 extern ExprBuiltinFunc *expr_builtin_funcs;
 extern const size_t     expr_n_builtin_funcs;
 extern ExprBuiltinVar  *expr_builtin_vars;
@@ -37,6 +48,8 @@ ExprError expr_set(Expr *e, const char *expr) __attribute__((warn_unused_result)
 ExprError expr_eval(Expr *e, double *out_res) __attribute__((warn_unused_result));
 void expr_set_var(Expr *e, const char *name, double val);
 bool expr_get_var(Expr *e, const char *name, double *out); /* Returns false if not present */
-void expr_set_func(Expr *e, const char *name, double (*func)(double *args), size_t n_args);
+void expr_set_func(Expr *e, const char *name, double (*func)(Expr *e, ExprArg *args), ExprArgType *arg_types, size_t n_args);
+void expr_set_userdata(Expr *e, void *userdata);
+void *expr_get_userdata(Expr *e);
 
 #endif /* __EXPR_H__ */