John Lindal’s Blog

Lisp

January 21, 2010 on 11:45 am | In Programming | No Comments

While driving to work this morning, I finally separated out what has been bothering me about Lisp. Functional programming proponents tend to make a big deal about avoiding side effects because it avoids long range coupling between functions and it makes list iteration trivially parallelizable. This is good for program maintenance and effective utilization of all the cores in a CPU.

Side effects are eliminated by avoiding (1) global variables and (2) functions that modify their arguments.

Point #1 is relatively easy in any language (except assembly), but it does require discipline, since global resources like files and databases are in principle always directly accessible, even if an encapsulating interface exists.

Point #2 is very difficult/painful in strongly typed languages like C++ and Java but very easy in untyped languages like Lisp, Perl, and JavaScript, mainly because untyped languages make it very easy to return a heterogeneous list of values/objects.

This is what has been bothering me about Lisp in the back of my mind: a list is the most natural return value, both because the syntax is simple and because most standard functions operate on lists. A homogeneous list is fine, of course, but a list of heterogenous values is terrible because (1) it is hard to remember what is returned in each slot and (2) without compile time checks, modifying a function to insert a new value into the returned list creates a maintenance nightmare. You have to manually find and update every use of the modified function, and if you miss one, you have a subtle bug.

How can we avoid all this trouble? Using a map instead of a list alleviates the problems because (1) well chosen key names are easier to remember and (2) most of the existing uses of the function probably will not need to be updated because they do not care about the new value and the original values will still be accessible via the same keys.

Unfortunately, working with maps (or hash tables) in Lisp is a lot messier than working with lists. In Perl and JavaScript maps are part of the language syntax.