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A command language on top of D

Til is a command language, like Tcl, built on top of D, so it kind of has a foot in both worlds.

Being a command language means it is easy to learn and the syntax is very extensible. And being built with D means it is very simple to build new modules for it.

Til makes use of Fibers so the interpreter is async by default and spawning a new process (“process” not in the OS sense, but in the Erlang sense) is trivial.

And, finally, Til has data streams that allow you to process data using a (hopefully) familiar concept: pipes.

(Oh, and Til is very opinionated. Beware.)

So, in brief, Til is



set x 1
set y 2
set z 3

Sure, it´s a bit unusual to use “set” instead of the much more common syntax x = 1 and to many it may feel too strange but if you can overcome this feeling you´ll see that it´s actually a very elegant way of setting values in a command language.

You see, one of the goals of the language is to have a coherent and simple enough syntax so that you´ll never be trapped in the “but there is more” cycle – instead, learn it once, learn it fast and expect no surprises after that.

Variables values and printing

set s "Hello, World!"
print $s

The snippet above will print Hello, World!. Til has the concept of Atoms, so in set s "Hello, World!" we have 2 Atoms (set and s) and 1 String ("Hello, World!"). And in print $s we have, again, 2 Atoms (print and $s), being the second one a Substitution Atom, that is, an Atom that, when evaluated, returns the value stored in the current context with name s.

ExecLists, Math and String substitutions

set a 11
set b 22
set result [math ($a + $b)]
print "The result is $result"

The snippet above will print The result is 33. The first new concept is the use of square brackets ([]). They form what we call an ExecList.

An ExecList contains any SubProgram and are evaluated immediately, that is, the SubProgram is executed before the command (in this case, set) is run. So, when saying set result [math ($a + $b)], math ($a + $b) will be executed and the result (33) will become the last argument of the set command, becoming set result 33.

The last item in this session is string substitution. It works as expected, really: you can reference values inside a string using the $ sign.


The following snippet come from the first version of Redis, when it was still called LMDB:

proc cmd_push {fd argv dir} {
    if {[catch {
        llength $::db([lindex $argv 1])
    }]} {

What is being said is “if there is something in the in-memory database slot whose key is given by the second item inside argv”. The problem is: phew!, that´s a lot of commands, specially for so common operations.

In Til there´s the concept of Extractions:

set d [dict (alfa 11) (beta 22) (gama 33)]
print "alfa is " <$d alfa>
# Output: alfa is 11

The <> syntax follows the pattern <data index> or <data range>. One can retrieve elements from a list easily:

set lista (a b c d e)
print "Second element is " <$lista 1>
# a
print "Fourth and fifth elements are " <$lista 3 5>
# (d e)
print "First element is " <$lista head>
# a
print "Tail is " <$lista tail>
# (b c d e)
print "Even-indexed elements are " <$lista (0 2 4)>
# (a c e)

This makes the code much cleaner and easier to understand. The example from LMDB could be written this way:

proc cmd_push {fd argv dir} {
    if ([list.length <$db <$argv 1>>] > 0) {

Procedures, SubLists and SimpleLists

proc f (x y) {
    return [math ($x * $y)]

In this example you can see how new “functions” are declared. Technically a procedure should not return a value but, you know… It´s so much easier to follow some traditions – for developers coming from Tcl the proc syntax will feel very familiar, so we go with it.

The parameters list, (x y) is a SimpleList. That means its content is not a SubProgram, but simply a list of “ListItems”. This content is not supposed to be executed as a program, but evaluated as a list.

Now, the procedure body is supposed to be executed at some time, so it´s declared as being a SubList, that is, a list of commands that represents a SubProgram. But, contrary to what happens with an ExecList, a SubList will not be executed immediately.

You see, Til is a command language. So proc is, in the end, just another command. It´s signature is proc name parameters body, in which:

So you must pass these “types” to the command or at least something that evauates to these types. If you store a SubList into a variable named body you could say:

proc f (x y) $body

And that´s okay, too. If your parameters list is stored into another variable, you also could say:

proc f $parameters_list $body

And, finally, if your procedure name is also stored into a variable, you coud also say:

proc $procedure_name $parameters_list $body

As long as each variable represents an Atom, a SimpleList and a SubList, you´ll be able to define a new procedure without any problem.

Ranges, foreach, comments and Streams

The simplest example showing the use of streams is this foreach that prints the current number from a range of some integer numbers:

range 3 | foreach x {
    print $x

# The expected output is:
# 0
# 1
# 2
# 3

(Yup, range will include the “limit” – “3” in this case.)

range is a very versatile command (it´s actually the module std.range) that allows you to create various kinds of ranges and even transform a SimpleList into a data stream.

Data streams travel through pipes (|) and various commands connected by pipes are called a Pipeline.

The rules governing the use of data streams are these:

So, for instance, if you want to walk through a directory you should use a data stream, since you usually cannot guarantee how many entries there are in the directory. And the same applies to data being received from a socket or read from a file.

Interestingly, foreach only works with data streams. And it only accepts one atom to name the current item. If your stream is composed of SimpleLists (that is: if each item is composed of multiple values) you can use destructuring set to store each one into a variable:

my_stream | foreach item {
    # $item is (origin headers data)
    set (origin headers data) $item

That is because the set command can be used to “destructure” a SimpleList:

set a b c (1 2 3)

Spawning processes, messages and backpressure

proc ping (target) {
    receive | foreach msg {
        send $target $msg

proc pong () {
    receive | foreach msg {
        print "Received $msg"

set writer_process [spawn pong]
set sender_process [spawn ping $writer_process]

send $sender_process "a message and exited"

The expected output of this program would be:

Received a message and exited

The summary of what happened is as follows:

About backpressure: each process has a message box with limited size. If your own process is trying to send a message to another one whose message box is already full, your process will be blocked.

(It will block its execution, but not the scheduler itself, of course. That is: other processes won´t be affected.)


There is a core concept in Til that is: every program is one file only. There is no “module” in the Python or Ruby sense, like an entity that lives somewhere inside the interpreter´s memory. In Til your modules are only commands providers and any Til code you would want to include into your program must be… included in your program.

Including a file has the same effect as copying its content and pasting where the include command is called.

The syntax is:

include "path/my_code.til"

You see, Til is a nice language, but you´re not supposed to write complex code in Til. If you want to implement some algorithm, write it in D and share it as a module.

The rationale is going to be explained in more details in another article, but for now it suffices to say that Til is a scripting language and you are not encouraged to write much more than “a program” in it, that is, it´s expected to be used as a “command and control” of other components, not as the main language implementing algorithms or data structures.

Errors and error handling

proc throw_error () {
    error "Test error"

proc on.error (x) {
    print "on.error:"
    print "  received: $x"
    print "  IGNORING IT!"

print "Calling procedure `throw_error`..."
print "Procedure `throw_error` was called and the error was handled."

And more

In the Til repository, look for the examples directory.

Building your own modules

See Building your own module.