A descriptive language describes the result to be computed and leaves
it up to the compiler to work out how to do it. This is often termed
"lazy" since one simple option for the compiler is to put off
computing things till really needed. Also the semantics of descriptive
languages typically require that nothing bad will happen if an
infinite (or failing) intermediate result is described as long as
there is no need to compute all of it. I think a descriptive language
has to be functional. Haskell is the relevant example.

A prescriptive language describes exactly how a result will be
calculated in a sequence of steps. Erlang is in this category, so it
is much easier for the traditional C/java programmer to get into. .

Scala can swing either way. However currently its libraries are
java-like or erlang-like. There is a wonderful series of blog postings
on how to monadify scala which points the way to writing scala
programs in the descriptive style: http://james-iry.blogspot.com/.

Descriptive style means using monads (or something moderately
equivalent) for interacting with external changing state. The whole
point of these is to restrict the compiler's options for
reorganization. Of course if it was done perfectly it would restrict
as much as necessary but no more. However I suspect that descriptive
compilers are going to have to learn to go through the same
optimization tricks as prescriptive languages to be really efficient
for dealing with monads: i.e. they will have to look at monadic
sequences of operations and say "the author wants these things done in
sequence but I can see that it can be reorganized without loss". I'd
love to hear that I'm wrong on this.

Still the main thing that seems to be relevant to efficiency for web
frameworks is a good system for efficiently fielding a large number of
very small communicating processes, as we see in Erlang and
(partially?) recreated with Scala actors. So a key step to moving to a
more descriptive style would seem to be introduce something like that
to haskell or monadified scala. Not that I am competent to make any
suggestions on how to do that.

The other distinctive "functional" common feature of erlang, haskell
and scala is pattern matching. This make for a nice programming style,
but I'm not sure if there is any deep significance?