futhark literate [options…] program


The command futhark literate foo.fut will compile the given program and then generate a Markdown file foo.md that contains a prettyprinted form of the program. This is useful for demonstrating programming techniques.

  • Top-level comments that start with a line comment marker (--) and a space in the next column will be turned into ordinary text in the Markdown file.

  • Ordinary top-level definitions will be enclosed in Markdown code blocks.

  • Any directives will be executed and replaced with their output. See below.

Warning: Do not run untrusted programs. See SAFETY below.

Image directives and builtin functions shell out to convert (from ImageMagick). Video and audio generation uses ffmpeg.

For an input file foo.fut, all generated files will be in a directory named foo-img. A file parameter passed to a directive may not contain a directory component or spaces.



The backend used when compiling Futhark programs (without leading futhark, e.g. just opencl). Defaults to c.


The program used to perform operations (eg. compilation). Defaults to the binary running futhark literate itself.


Override the default output file. The image directory will be set to the provided FILE with its extension stripped and -img/ appended.


Pass an option to benchmark programs that are being run. For example, we might want to run OpenCL programs on a specific device:

futhark literate prog.fut --backend=opencl --pass-option=-dHawaii

Pass an extra option to the compiler when compiling the programs.


Do not run the compiler, and instead assume that the program has already been compiled. Use with caution.


Terminate immediately without producing an output file if a directive fails. Otherwise a file will still be produced, and failing directives will be followed by an error message.

-v, --verbose

Print verbose information on stderr about directives as they are executing. This is also needed to see #[trace] output.


A directive is a way to show the result of running a function. Depending on the directive, this can be as simple as printing the textual representation of the result, or as complex as running an external plotting program and referencing a generated image.

Any directives that produce images for a program foo.fut will place them in the directory foo-img/. If this directory already exists, it will be deleted.

A directive is a line starting with -- >, which must follow an empty line. Arguments to the directive follow on the remainder of the line. Any expression arguments are given in a very restricted subset of Futhark called FutharkScript (see below).

Some directives take mandatory or optional parameters. These are entered after a semicolon and a linebreak.

The following directives are supported:

  • > e

    Shows the result of executing the FutharkScript expression e, which can have any (transparent) type.

  • > :video e[; parameters...]

    Creates a video from e. The optional parameters are lines of the form key: value:

    • repeat: <true|false>

    • fps: <int>

    • format: <webm|gif>

    • file: <name>. Make sure to provide a proper extension.

    e must be one of the following:

    • A 3D array where the 2D elements is of a type acceptable to :img, and the outermost dimension is the number of frames.

    • A triple (s -> (img,s), s, i64), for some types s and img, where img is an array acceptable to :img. This means not all frames have to be held in memory at once.

  • > :brief <directive>

    The same as the given directive (which must not start with another >), but suppress parameters when printing it.

  • > :covert <directive>

    The same as the given directive (which must not start with another >), but do not show the directive itself in the output, only its result.

  • > :img e[; parameters...]

    Visualises e. The optional parameters are lines of the form key: value:

    • file: NAME. Make sure to use a proper extension.

    The expression e must have one of the following types:

    • [][]i32 and [][]u32

      Interpreted as ARGB pixel values.

    • [][]f32 and [][]f64

      Interpreted as greyscale. Values should be between 0 and 1, with 0 being black and 1 being white.

    • [][]u8

      Interpreted as greyscale. 0 is black and 255 is white.

    • [][]bool

      Interpreted as black and white. false is black and true is white.

  • > :plot2d e[; size=(height,width)]

    Shows a plot generated with gnuplot of e, which must be an expression of type ([]t, []t), where t is some numeric type. The two arrays must have the same length and are interpreted as x and y values, respectively.

    The expression may also be a record expression (not merely the name of a Futhark variable of record type), where each field will be plotted separately and must have the type mentioned above.

  • > :gnuplot e; script...

    Similar to plot2d, except that it uses the provided Gnuplot script. The e argument must be a record whose fields are tuples of one-dimensional arrays, and the data will be available in temporary files whose names are in variables named after the record fields. Each file will contain a column of data for each array in the corresponding tuple.

    Use set term png size width,height to change the size to width by height pixels.

  • > :audio e[; parameters...]

    Creates a sound-file from e. The optional parameters are lines of the form key:value:

    • sampling_frequency: <int>

      The sampling frequency (in Hz) of the input. Defaults to 44100.

    • codec: <name>

      The codec of the output. Defaults to wav. Other common options include mp3, flac, ogg and opus.

    The expression e must have one of the following types:

    • []i8 and []u8

      Interpreted as PCM signed/unsigned 8-bit audio.

    • []i16 and []u16

      Interpreted as PCM signed/unsigned 16-bit audio.

    • []i32 and []u32

      Interpreted as PCM signed/unsigned 32-bit audio.

    • []f32 and []f64

      Interpreted as PCM signed/unsigned 32/64 bit floating-point audio. Should only contain values between -1.0 and 1.0.

    For each type of input, it is also possible to give expressions with a two-dimensional type instead, e.g. [][]f32. These expressions are interpreted as an array of channels, making it possible to do stereo audio by returning e.g. [2][]f32. For stereo output, the first row is the left channel and the second row is the right channel. This functionality uses the amerge filter from ffmpeg, so consult the documentation there for additional information.


Only an extremely limited subset of Futhark is supported:

script_exp  ::=    fun script_exp*
                 | "(" script_exp ")"
                 | "(" script_exp ( "," script_exp )+ ")"
                 | "[" script_exp ( "," script_exp )+ "]"
                 | "empty" "(" ("[" decimal "]" )+ script_type ")"
                 | "{" "}"
                 | "{" (id = script_exp) ("," id = script_exp)* "}"
                 | "let" script_pat "=" script_exp "in" script_exp
                 | literal
script_pat  ::=   id | "(" id ("," id) ")"
script_fun  ::=   id | "$" id
script_type ::=  int_type | float_type | "bool"

Note that empty arrays must be written using the empty(t) notation, e.g. empty([0]i32).

Function applications are either of Futhark functions or builtin functions. The latter are prefixed with $ and are magical (usually impure) functions that could not possibly be implemented in Futhark. The following builtins are supported:

  • $loadimg "file" reads an image from the given file and returns it as a row-major [][]u32 array with each pixel encoded as ARGB.

  • $loaddata "file" reads a dataset from the given file. When the file contains a singular value, it is returned as value. Otherwise, a tuple of values is returned, which should be destructured before use. For example: let (a, b) = $loaddata "foo.in" in bar a b.

  • $loadbytes "file" reads the contents of the given file as an array of type []u8.

  • $loadaudio "file" reads audio from the given file and returns it as a [][]f64, where each row corresponds to a channel of the original soundfile. Most common audio-formats are supported, including mp3, ogg, wav, flac and opus.

FutharkScript supports a form of automatic uncurrying. If a function taking n parameters is applied to a single argument that is an n-element tuple, the function is applied to the elements of the tuple as individual arguments.


Some directives (e.g. :gnuplot) can run arbitrary shell commands. Other directives or builtin functions can read or write arbitrary files. Running an untrusted literate Futhark program is as dangerous as running a shell script you downloaded off the Internet. Before running a program from an unknown source, you should always give it a quick read to see if anything looks fishy.


FutharkScript expressions can only refer to names defined in the file passed to futhark literate, not any names in imported files.


futhark-script, futhark-test, futhark-bench