Hello ! Frequently, our C program computes a set of approximately 50 values that it wants to make accessible for a user defined call back so that the user can choose to print some of these values (if some condition is met for instance).
I first thought to make some C functions accessible that would return the values (one C getter per value), and have the C scheme procedure call these C functions for the values it's interested in. But the problem is that it's a little verbose (both in C and in scheme due to the required parenthesis). I then though of binding the values to global variables that the user callback would use but ruled this out because I was afraid this solution would be slow. Then I wanted to pass the values as regular parameters, but I don't want to impose the user to define his function with 50 parameters. So finally we came up with something like this : The user enter merely a list of the values he wants (in a "configuration" file) : (define user-fields `(foo bar (+ baz foo) (if (> foo bar) 1 2))) And then in C we evaluate : (define (hook-helper %s) (lambda () #\t)) where %s is the long list of parameters (foo bar baz...) that's inserted by the C program. And : (define (hook . args) (local-eval (cons print-user-fields user-fields) (procedure-environment (apply hook-helper args)))) So the user entry is minimal and we can, for each set of values, apply the hook function to the values (which is I think one of the fastest way to make available some a set of named values to scheme). The downside is : the above scheme line looks like black magic. Can anyone suggest a better way to do this ?
