5.4.5  Mythryl Functions: Value Capture

“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:


    fun delayed_print  string
        \\ () = 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

What has happened here is that the anonymous functions (thunk)s constructed by the \\ () = printf "%s\n" string; line of code are capturing the string arguments visible to them.

The Mythryl
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
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:


    Selector = FIRST | SECOND;

    fun cons (a, b)
        \\ selector = case selector
                      FIRST  => a;
                      SECOND => b;

    x = cons( "abc", "def" );

    printf "%s\n" (x FIRST);
    printf "%s\n" (x SECOND);

When run, the above yields:

    linux$ ./my-script

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.

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