# 2. Basic Usage¶

Futhark contains several code generation backends. Each is provided as a full standalone compiler binary. For example, futhark-c compiles a Futhark program by translating it to sequential C code, while futhark-pyopencl generates Python and the PyOpenCL library. The different compilers all contain the same frontend and optimisation pipeline - only the code generator is different. They all provide roughly the same command line interface, but there may be minor differences and quirks due to characteristics of the specific backends.

## 2.1. Compiling to Executable¶

A Futhark program is stored in a file with the extension .fut. It can be compiled to an executable program as follows:

\$ futhark-c prog.fut


This makes use of the futhark-c compiler, but any other will work as well. The result is an executable binary called prog. If we had used futhark-py instead of futhark-c, the prog file would instead have contained Python code, along with a shebang for easy execution. In general, when compiling file foo.fut, the result will be written to a file foo (i.e. the extension will be stripped off). This can be overridden using the -o option.

When a Futhark program is run, execution starts at the function named main. If the program has no main function, the compiler will fail with an error. If main takes any parameters, these will be read from standard input in Futhark syntax. Note: Tuple value syntax is not supported. Instead, pass the values comprising the tuple as if they were immediate parameters to the main function. We recommended using only primitive types and arrays of primitive types as parameter (and return) types in the main function.

Instead of compiling, there is also an interpreter, futharki. Be aware that it is very slow, and does not produce better error messages than the compiler. Note: If you run futharki without any options, you will see something that looks deceptively like a REPL, but it is not yet finished, and useless in its present state.

## 2.2. Compiling to Library¶

While compiling a Futhark program to an executable is useful for testing, it is not the intended use case. Instead, a Futhark program should be compiled into a reusable library in some target language, enabling integration into a larger program. At the moment, this work has only been done for the futhark-py and futhark-pyopencl backends, and only the latter of these generates sufficiently performant code for it to be worthwhile.

We can use futhark-pyopencl to translate the program futlib.fut into a Python module futlib.py with the following command:

futhark-pyopencl --library futlib.fut


This will create a file futlib.py, which contains Python code that defines a class named futlib. This class defines one method for each entry point function (see Entry Points) in the Futhark program. After the class has been instantiated, these methods can be invoked to run the corresponding Futhark function. The reason for why the module does does not expose functions directly, is that some complicated initialisation and stateful bookkeeping may be necessary for sophisticated backends, such as PyOpenCL.