“You are full of surprises, Mr Baggins!.”
—Gimli son of Gloin
One will often see C code in which a function is passed around together with its argument. Together they constitute a fate, a suspended computation which may be continued at any time by calling the function with its argument.
This arrangement is necessary in C because C functions are fixed at compile time, immutable at run time: The only way to express a pending computation is to specify the code and state separately.
Mythryl allows the two to be neatly combined:
#!/usr/bin/mythryl
fun delayed_print string
=
fn () = printf "%s\n" string;
fate_a = delayed_print "Just";
fate_b = delayed_print "another";
fate_c = delayed_print "Mythryl";
fate_d = delayed_print "hacker!";
fate_a ();
fate_b ();
fate_c ();
fate_d ();
When run this produces
linux$ ./my-script
Just
another
Mythryl
hacker!
linux$
What has happened here is that the anonymous functions (thunk)s constructed
by the fn () = printf "%s\n" string; line of code are capturing the
string arguments visible to them.
The Mythryl fn statement is actually a data constructor!
In principle, if we really wanted to, we could write our programs constructing all of our datastructures entirely in terms of fn statements capturing value, although the resulting code would not be very pleasant to read. For example, we could constructs lists via a function which captures two values and returns a thunk capable of returning either on request. Here is a slightly simplified example of such a function:
#!/usr/bin/mythryl
Selector = FIRST | SECOND;
fun cons (a, b)
=
fn selector = case selector
FIRST => a;
SECOND => b;
esac;
x = cons( "abc", "def" );
printf "%s\n" (x FIRST);
printf "%s\n" (x SECOND);
When run, the above yields:
linux$ ./my-script
abc
def
linux$
Re-inventing the Mythryl list this way makes no sense, but using value capture to construct fates for later execution makes a lot of sense. You will see this pervasively in production Mythryl code that you read, and after using this technique for awhile you will wonder how you ever programmed without it.