Nix, a purely functional language best known for describing reproducible systems, seems to have just enough power to model imperative behavior. This article walks through a small experiment: an embedded DSL in Nix that lets you write programs with imperative features, such as mutable variables, loops, exceptions, functions, and I/O.

This article is an expansion of my earlier forum post about writing an interactive program in Nix.

Assign and print

Monads are the standard abstraction for mutation of states. A monad wraps values with extra information (such as side effects). A monadic operation can be seen as a function from some state to a new state, possibly with side effects encoded as data rather than real I/O.

This DSL adopts that idea. Each statement is a function of a state that returns an instruction of how to mutate the state. Consider this do function that “executes” a statement:

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do = statement: state: state // statement state state;

Each statement receives the current state and returns a function that mutates the current state into an updated one. It is equivalent to a monadic bind, but the monads are very trivial here because the unit operation is just the identity function.

Let us define some simple statements such as assignment and printing to the console:

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#!/usr/bin/env nix-build

let
	do = statement: state: state // statement state state;
	assign = variables: state: state // variables;
	print = builtins.trace;

	state0 = { };
	state1 = do (state: assign { i = 1; }) state0;
	state2 = do (state: print "Current value: ${toString state.i}") state1;
	state3 = do (state: assign { i = state.i + 1; }) state2;
	state4 = do (state: print "Current value: ${toString state.i}") state3;

in builtins.seq state4 { }

The final builtins.seq call forces the final state to be evaluated so that all the statements will be executed. There is no need for a builtins.deepSeq because any data that we wish to see via print will be force evaluated. The output:

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trace: Current value: 1
trace: Current value: 2

This is nice! We can slightly modify do to make it handle a list of statements instead so that we do not have to call do for each statement. We can also rename state to just _ so that the code does not look too verbose.

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#!/usr/bin/env nix-build

let
	do = statements: _:
		if statements == [ ] then _
		else do (builtins.tail statements) (_ // builtins.head statements _ _);
	assign = variables: _: _ // variables;
	print = builtins.trace;

in builtins.seq (do [
	(_: assign { i = 1; })
	(_: print "Current value: ${toString _.i}")
	(_: assign { i = _.i + 1; })
	(_: print "Current value: ${toString _.i}")
] { }) { }

This gives the same output as before. I used a recursion to iterate over the list of statements instead of just using builtins.foldl' because this form is easier to be modified later to incorporate exception handling.

Exception handling

Next, we add exception handling to the DSL. I introduce it before control flow because I will make loops rely on exceptions to implement break and continue.

For simplicity, I will make throwing an exception just setting a special variable _thrown_ in the state:

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throw = thrown: assign { _thrown_ = thrown; };

(Notice that this shadows the built-in throw function, so we will refer to the built-in one as builtins.throw if needed.) In do, we need to check whether an exception has been thrown after each statement. If so, we stop executing further statements.

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do = statements: _:
	if statements == [ ] then _
	else let result = _ // builtins.head statements _ _; in
		if builtins.hasAttr "_thrown_" result then result
		else do (builtins.tail statements) result;

Then, in the try statement, we check for _thrown_ after executing the try block. If an exception was thrown, we execute the catch block with the exception value passed to it (after unsetting the _thrown_ variable).

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try = statements: catch: _: let result = do statements _; in
	if builtins.hasAttr "_thrown_" result then
		do (catch result._thrown_) (removeAttrs result [ "_thrown_" ])
	else result;

With these definitions, we can now write code that throws and catches exceptions:

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let # ...
in builtins.seq (do [
	(_: try [
		(_: throw "Some error message.")
	] (exception: [
		(_: print "Caught exception: ${exception}")
	]))
] { }) { }

Control flow

The control flow statements include if and while, and the latter is accompanied by break and continue. Both of them needs a condition expression that evaluates to a boolean value. The while condition will be evaluated multiple times for different states, so it at least needs to be a function that maps the state to a boolean instead of just a boolean. While the if condition can be a simple boolean expression, I will make it a function as well for consistency.

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if' = condition: trueStatements: falseStatements: _:
	do (if condition _ then trueStatements else falseStatements) _;
whileWithoutJump = condition: body:
	if' condition [ body (_: whileWithoutJump condition body) ] [ ];

Now, to implement break and continue, we can use exception handling. Define them as throwing special exception values:

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break = throw "_break_";
continue = throw "_continue_";

Then, define while to use try to catch these exceptions. The "_continue_" exception is caught inside the loop body, and the "_break_" exception is caught outside the loop to terminate the loop.

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catchOnly = handled: thrown: if thrown == handled then [ ] else [ (_: throw thrown) ];
while = condition: statements: try [
	(_: whileWithoutJump condition (_: try statements (catchOnly "_continue_")))
] (catchOnly "_break_");

We can now write some loops.

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let # ...
in builtins.seq (do [
	(_: assign { i = 0; })
	(_: while (_: _.i < 5) [
		(_: if' (_: _.i == 4) [
			(_: break)
		] [ ])
		(_: if' (_: _.i == 2) [
			(_: assign { i = _.i + 1; })
			(_: continue)
		] [ ])
		(_: print "Current i: ${toString _.i}")
		(_: assign { i = _.i + 1; })
	])
] { }) { }

This prints:

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trace: Current i: 0
trace: Current i: 1
trace: Current i: 3

Functions

Defining functions is already possible without any special constructs, but only assign and do is needed:

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let # ...
in builtins.seq (do [
	(_: assign { hello = name: do [
		(_: print "Hello, ${name}!")
	]; })
	(_: _.hello "world")
] { }) { }

We can make it nicer by having it support return to exit early from the function. Similar to break and continue, we can implement return using exceptions.

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return = throw "_return_";
function = functions: assign (builtins.mapAttrs (name: fun: arg: try (fun arg) (catchOnly "_return_")) functions);

Then, we can define functions with return support:

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let # ...
in builtins.seq (do [
	(_: function { hello = arg: [
		(_: if' (_: arg == "") [
			(_: print "What's your name?")
			(_: return)
		] [ ])
		(_: print "Hello, ${arg}!")
	]; })
	(_: _.hello "")
	(_: _.hello "world")
] { }) { }

This prints:

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trace: What's your name?
trace: Hello, world!

Input

We can use builtins.readFile to read input from files. To read from stdin, we need to use Lix to do that because the vanilla implementation of Nix does not support builtins.readFile /dev/stdin.

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let # ...
	read = builtins.readFile;
in builtins.seq (do [
	(_: assign { input = read /dev/stdin; })
	(_: print "You entered: ${_.input}")
] { }) { }

Because builtins.readFile reads the entire file until EOF, when you finish input, you need to send an EOF signal, typically by pressing Ctrl+D on an empty line. Notice that you need to press this combination on a empty line, so the input you get in Nix probably ends with a newline character that you want to trim.

If you want to read from stdin multiple times, to ensure that the Nix interpreter tries to reopen /dev/stdin each time, you must not write input = builtins.readFile /dev/stdin; in the first let block and refer to input every time you want to read from stdin. Instead, you should use read /dev/stdin every time, or wrap it in a function and call the function every time.

Other minor improvements

We may want to separate all those things in the first let block into a separate file, which let us call imperative.nix, so that we do not need to write out all those things like assign, print every time we write an imperative program. However, this would still mean that we have to pass a bunch of variables like this:

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let
	inherit (import ./imperative.nix) do assign print throw;
	# ...
in builtins.seq (do [
	# ...
] { }) { }

One way to avoid this is to use builtins.scopedImport in imperative.nix, but in my opinion, a better approach is to simply put all those things in the initial state _. Now, the actually executable Nix file just contain the list of statements, and imperative.nix will import it to run it. All the keywords like while and assign become variables in _, so nothing needs to be imported or scoped in the executable Nix file. However, this would also mean that we need to import the program in imperative.nix instead of the other way around. To make imperative.nix know which file to import, we can provide it with an argument, so it looks like this:

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{ input }: let # ...
in builtins.seq (
	do
	(import (if builtins.match input "^/" != null then input else "${toString ./.}/${input}"))
	{ inherit assign if' while print throw break continue return function read try; }
) { }

To run a program in program.nix, you need to run nix-build imperative.nix --argstr input program.nix. You may encapsulate it in a shebang in imperative.nix:

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#!/usr/bin/env nix-shell
#!nix-shell --pure -i "nix-build --no-out-link" -p lix

If you now make imperative.nix executable, you can run a program using imperative.nix --argstr input program.nix.

This now enables us to implement an import function that runs programs from other files:

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import = file: do (builtins.import file);

(This shadows the built-in import function, so we will refer to the built-in one as builtins.import if needed.) You can now do something like this:

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# program.nix
[
	(_: _.import ./other-program.nix)
]
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# other-program.nix
[
	(_: _.print "Hello from other program!")
]

When you run imperative.nix --argstr input program.nix, it will print:

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trace: Hello from other program!

We may improve it further by removing the need of --argstr input and also adding support for passing command line arguments to the program, at the cost of having to write a little bit of shell script and jq in the shebang (so really we cannot call it a “purely Nix” implementation now, but the nix-shell shebang is simply too powerful):

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#!/usr/bin/env nix-shell
#!nix-shell --pure -i "sh -c '_1=\$1; _2=\$2; shift 2; exec nix-build --no-out-link \"\$_1\" --argstr input \"\$_2\" --argstr argvJSON \"\$(printf \"%s\\\\0\" \"\$@\" | jq -Rsc \"split(\\\"\\\\u0000\\\")[:-1]\")\"' --" -p lix jq

{ input, argvJSON }: let # ...
	argv = builtins.fromJSON argvJSON;
in builtins.seq # ...

The mechanism is that the shell script in the shebang extracts the first two command line arguments as the Nix file to run and the input file, and the rest of the command line arguments are concatenated with null characters and converted to a JSON array using jq. In imperative.nix, we parse the JSON array back to a Nix list and provide it as the variable argv in the initial state _. You can now simply use a shebang #!/usr/bin/env imperative.nix in program.nix, and then you can run it with ./program.nix arg1 arg2 if you make it executable, and the command line arguments will be available in the list _.argv.

Finally, we can use try instead of do in the ultimate builtins.seq call to catch any uncaught exceptions in the program. Here is the final version of imperative.nix and an example program.nix that uses most of the features we introduced:

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#!/usr/bin/env nix-shell
#!nix-shell --pure -i "sh -c '_1=\$1; _2=\$2; shift 2; exec nix-build --no-out-link \"\$_1\" --argstr input \"\$_2\" --argstr argvJSON \"\$(printf \"%s\\\\0\" \"\$@\" | jq -Rsc \"split(\\\"\\\\u0000\\\")[:-1]\")\"' --" -p lix jq

{ input, argvJSON }: let
	do = statements: _:
		if statements == [ ] then _
		else let result = _ // builtins.head statements _ _; in
			if builtins.hasAttr "_thrown_" result then result
			else do (builtins.tail statements) result;
	assign = variables: _: _ // variables;
	read = builtins.readFile;
	print = builtins.trace;
	if' = condition: trueStatements: falseStatements: _:
		do (if condition _ then trueStatements else falseStatements) _;
	whileWithoutJump = condition: body:
		if' condition [ body (_: whileWithoutJump condition body) ] [ ];
	catchOnly = handled: thrown: if thrown == handled then [ ] else [ (_: throw thrown) ];
	while = condition: statements: try [
		(_: whileWithoutJump condition (_: try statements (catchOnly "_continue_")))
	] (catchOnly "_break_");
	throw = thrown: assign { _thrown_ = thrown; };
	break = throw "_break_";
	continue = throw "_continue_";
	return = throw "_return_";
	try = statements: catch: _: let result = do statements _; in
		if builtins.hasAttr "_thrown_" result then
			do (catch result._thrown_) (removeAttrs result [ "_thrown_" ])
		else result;
	function = functions: assign (builtins.mapAttrs (name: fun: arg: try (fun arg) (catchOnly "_return_")) functions);
	import = file: do (builtins.import file);
	argv = builtins.fromJSON argvJSON;
in builtins.seq (
	try
	(builtins.import (if builtins.match input "^/" != null then input else "${toString ./.}/${input}"))
	builtins.throw
	{ inherit assign if' while print throw break continue return function read try import builtins argv; }
) { }
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#!/usr/bin/env imperative.nix

[
	(_: _.import (_.builtins.toFile "other-input.nix" "[ (_: _.print \"Hello from other file!\") ]"))
	(_: _.function { hello = arg: [
		(_: _.if' (_: arg == "") [
			(_: _.print "What's your name?")
			(_: _.return)
		] [ ])
		(_: _.print "Hello, ${arg}!")
	]; })
	(_: _.hello "")
	(_: _.hello "World")
	(_: _.while (_: _.argv != [ ]) [
		(_: _.hello (_.builtins.head _.argv))
		(_: _.assign { argv = _.builtins.tail _.argv; })
	])
	(_: _.assign { i = 0; })
	(_: _.while (_: _.i < 5) [
		(_: _.if' (_: _.i == 4) [
			(_: _.break)
		] [ ])
		(_: _.if' (_: _.i == 2) [
			(_: _.assign { i = _.i + 1; })
			(_: _.continue)
		] [
			(_: _.hello "${_.builtins.toString _.i}")
		])
		(_: _.assign { i = _.i + 1; })
	])
	(_: _.try [
		(_: _.throw "Some error message.")
	] (exception: [
		(_: _.print "Caught exception: ${exception}")
	]))
	(_: _.print "Goodbye!")
]

When you run ./program.nix Kat "Ulysses Zhan", it will print:

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trace: Hello from other file!
trace: What's your name?
trace: Hello, World!
trace: Hello, Kat!
trace: Hello, Ulysses Zhan!
trace: Hello, 0!
trace: Hello, 1!
trace: Hello, 3!
trace: Caught exception: Some error message.
trace: Goodbye!

Possible improvements

Maybe I can improve it further by adding support for local variables. It would also be nice if functions with side effects can also have return values, but this would be tricky to implement.