Motivation for compile time function execution

[Walter Bright]

The sqrt() example I gave is cute, but not a very interesting reason for 
doing compile time function execution. So what's the real reason?

A while back, Eric Anderton and Don Clugston showed how one could use 
template metaprogramming techniques to do, for example compile time 
regular expressions. It was a marvelous technical demonstration, and 
showed that essentially any computation could be done, using templates, 
at compile time.

There were some serious problems, though:

1) They are hard and unintuitive to write (for those not comfortable 
with functional programming, which is most of us).

2) The result looks awful - templates are just syntactically unsuited to 
this, even though they're a lot easier on the eye than C++ template 
metaprograms.

3) This is off-putting enough that some question even having such 
facilities in the language, as it results in impenetrable code 
unmaintainable by joe coders.

4) While theoretically capable of any computation, such template 
metaprogramming had severe practical limitations. Every step of every 
computation required generating a unique template. This naturally is 
incredibly slow and memory consumptive (C++ metaprogramming is notorious 
for taking all night to do a build). Even worse, if you're going to use 
string templates to parse, say, a 1000 character DSL, the template name 
generated must include its arguments, so that template identifier will 
be 1000 characters long. Then, it slices off the first character, and 
generates another template for the rest (999), then 998, then 997, etc., 
until 1000 templates are generated averaging 500 characters long. It 
doesn't take much of that before the whole thing collapses.

5) In casting about for a solution, the compile time regex came up 
again. This was still strongly disliked.

6) I promised to try and make template metaprogramming less obtuse, and 
what better way to do that than to obsolete a whole class of templates, 
replacing them with ordinary functions?