Basic usage with the factorial function
The factorial of a
number n, written n!, is the product of all integers from 1 to
n inclusive. It is often used as the hello world program of
functional languages. Usually, the given definition is recursive,
but Futhark does not support recursion, so instead we use
reduce:
def fact (n: i32): i32 = reduce (*) 1 (1...n)If we want this function to be usable from the outside world, we
need to define an entry point that calls it. By default, any
function by the name main is an entry point:
def main (n: i32): i32 = fact nThe function call fact n creates an array of the integers
1...n, then computes the product of all elements in the array.
The Futhark compiler employs loop fusion to remove the need for
the intermediate array to be actually created. Technically, fact n does not compute n!, but rather n! mod 2³², as i32s are
32 bit in size and will rapidly overflow for large n.
If we put the above program in a file fact.fut, we can compile it
using the OpenCL backend as such:
$ futhark opencl fact.futIf all goes well, this produces an executable program fact in the
current directory. Similarly, we can compile to sequential C code
with futhark c. Futhark is not intended to be used for writing
standalone programs, but it is supported in order to enable testing
and benchmarking. A standalone program will expect to be given its
arguments on standard input::
$ echo 2000000000 | ./factThis will write the result on standard output (0i32 - a 32-bit
zero).