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Does Lisp Influence Your C++?

Fri Jul 26, 2013

The idea for this post started on Aug 11, 2011, when I read Gregg’s post about choosing a scripting language for 3DScript, the CGM interactive test bed.  That article had a lot of good ideas, but I feel like it kind of missed the point regarding Lisp/Scheme/Functional Programming.  He used a do loop to try to make the point that the syntax is rather goofy.   The reason scheme came up at all is that it is used as a scripting language for acis3dt.exe our 3D ACIS Modeler Test application.

I actually agree that the syntax for a do loop is goofy.  However, learning about Lisp (and the lispy features of perl, see has made me a much better programmer than I would otherwise be.  In some ways, this thesis is older than the hills. To illustrate this, Google various subjects surrounding Lisp, Functional Programming, etc., for example, map-reduce.  When I talk with other programmers, I get the feeling that the message bears repeating.

So please bear with me while I sketch a few specifics.  Also, feel free to add your own in the comments:

  • Referential transparency makes things a lot easier to reason about.  Put another way: it is much easier to deal with C/C++ code that doesn’t modify global and static variables.
  • The best way to speed up code is to get a smarter algorithm. 

- Writing programs as compositions of simple (but high order functions), makes it much easier to reason about them:
- There are a few functional programming clichés that often help in dramatically speeding things up
                         - Memoization
                         - Make the algorithm lazy

  • Mathematical proofs are very easily given in terms of induction/recursion

- The literature on Lisp has lots of discussion about how you can convert between recursion and iteration.

  • Thinking about classes in terms of closures is helpful to me.

A common anti-pattern in legacy code is monolithic and large functions which give a detailed to do list.  Typically, I end up extracting functions from the huge monster, turning them into classes, and then parameterizing their behavior.  This is really almost the same thing as taking a code snippet, making a closure over the variables you need to encapsulate, etc.

C/C++  still tends to be ubiquitous because:
(a) So many system libraries are written in C or C++
(b) Well written C++ can be very fast

However, C-style languages don’t lend themselves to simple and concise reasoning about code.  If there were a well-established Lisp with a good linker, it might eat C’s lunch.

Here's a cartoon on the subject

What do you think?

Tags: 3D Modeling