Here comes a naive benchmark for C, Java, Erlang, Prolog and Haskell.
The benchmark is to compute Pythagorean triples.
The triple consists of three positive integers a, b, and c, such that a*a + b*b = c*c and additionally a+b+c<=n, where n=500.

Haskell:

f :: Int -> [ (Int, Int, Int) ]
f n = [ (x, y, z) |
x <- [ 1 .. n ],
y <- [ 1 .. n ],
z <- [ 1 .. n ],
x + y + z <= n,
(x * x) + (y * y) == (z * z) ]

Prolog:

pytriangle(N,A,B,C):-
between(1,N,A),
between(1,N,B),
between(1,N,C),
A + B + C =< N ,
A*A + B*B =:= C*C.

Erlang:

-module(test).
-export([pythag/1]).
pythag(N) ->
[ {A,B,C} ||
A <- lists:seq(1,N),
B <- lists:seq(1,N),
C <- lists:seq(1,N),
A+B+C =< N,
A*A+B*B =:= C*C
].

C:

int main(int argc,char *argv[]){
int a,b,c,n;
n=atoi(argv[1]);
for (a=1; a<=n; a++) {
for (b=1; b<=n; b++) {
for (c=1; c<=n; c++) {
if ((a+b+c <= n) && (a*a + b*b == c*c)){
printf("%d,%d,%d;",a,b,c);
}
}
}
}
return 0;
}

Java:

public class triangles
{
public static void main(String [] args)
{
int a,b,c,n;
n=Integer.parseInt(args[0]);
for (a=1; a<=n; a++) {
for (b=1; b<=n; b++) {
for (c=1; c<=n; c++) {
if ((a+b+c <= n) && (a*a + b*b == c*c)){
System.out.printf("%d,%d,%d;",a,b,c);
}
}
}
}
}
}

Here comes the results (in seconds).

Haskell (GHCi, version 6.12.1): 380

Haskell compiled (GHCi, version 6.12.1): 32

Prolog (SWI Prolog 5.7.4): 197.2

Erlang (R13B03): 59.5

Erlang HiPE (High Performance Erlang), native: 13.2

C (gcc 4.4.3): 0.8

Java (IcedTea6 1.8.1): 2.2 (with output redirected to /dev/null: 1.3)

The winner was obvious from the begining. But it's quite interesting how the declarative languages perform... hmmm. Kudos to Erlang.

By the way, have you noticed that the code is far better readable with the declarative approach?