5.3.18  Mythryl eval Operators

Scripting languages such as Perl frequently implement an eval operator allowing execution of source code strings dynamically created by the running script. This has a variety of handy uses ranging from implementing systems which interactively execute user-entered code to system which dynamically compile special-case code at need.

Mythryl implements a supported eval operator for accessing incremental compilation functionality:

    linux> my

    eval:  evali "2 + 2";


Perl and bash, being dynamically typed, are not bothered by the fact that the type of eval depends entirely upon the contents of its string argument.

In a language like Mythryl this ill-typedness is more problematic. This is not an unsurmountable problem. If it were, Mythryl’s interactive mode would not be able to print out the values of interactively entered expressions. But the solution is not something you would want to examine immediately before a meal.

Eventually, eval should be tweaked to have type String -> X where X can change from invocation to invocation. (Implementing this might be a nice undergrad compiler course project. Email me a patch and I’ll merge it in!)

For the moment, at least, Mythryl’s solution is just to supply in the library a half dozen odd statically typed eval variants covering common cases:

    eval:   String -> Void;

    evali:  String -> Int;
    evalf:  String -> Float;
    evals:  String -> String;

    evalli: String -> List( Int    );
    evallf: String -> List( Float  );
    evalls: String -> List( String );

Additional variants may be implemented as needed by cloning and tweaking the existing routines.

This isn’t a great solution, but it is much better than nothing!

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