# Language Reference Manual¶

This is the Felix Language Reference manual, it is intended primarily to document the common language interface presented to the programmer. It is not complete or precise because the grammar and features the user would normally call a language are actually defined in user space, in the library.

This chapter briefly explains some of the central concepts of Felix.

## Simplicity, Performance.¶

Felix is, first and foremost, dedicated to obtaining run time performance.

Felix motto is hyperlight performance which means we aim to run programs faster than C.

fun ack(x:int,y:int):int =>
if x == 0 then y + 1
elif y == 0 then ack(x - 1, 1)
else ack(x - 1, ack(x, y - 1))
endif
;

do
val n = 13;
var v = ack(3,n);
println$f"Ack(3,%d): %d" (n, v); done fun Tak (x:double, y:double, z:double): double => if (y >= x) then z else Tak(Tak(x - 1.0,y,z), Tak(y - 1.0,z,x), Tak(z - 1.0,x,y)) endif ; do val n = 12.0; var v = Tak(n*3.0, n*2.0, n*1.0); println$ f"%.2f" v;
done


To run it you just say:

flx test.flx


It’s pretty simple. Felix runs programs like Python does, you run the source code directly.

All the generated files are cached in the .felix/cache subdirectory of your \$HOME directory. Felix can run script files in read-only directories.

Felix translates the code into C++, compiles the C++, and runs it. Felix programs run fast. Felix itself implements high level optimisations beyond the scope of traditional compilers, then passes the generated code to your system C++ compiler which in turn implements low level optimisations.

Here’s a silly comparison for Ackermann’s function and Takfp, times in seconds:

Compiler Ack Takfp
Felix/clang 3.71 6.23
Clang/C++ 3.95 6.29
Felix/gcc 2.34 6.60
Gcc/C++ 2.25 6.25
Ocaml 2.93 8.41