Language Constructs - Lix Reference Manual

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# Language Constructs

## Recursive sets

Recursive sets are like normal attribute sets, but the attributes can refer to each other.

> rec-attrset = `rec {` [ name `=` expr `;` `]`... `}`

Example:

    rec {
      x = y;
      y = 123;
    }.x

This evaluates to `123`.

Note that without `rec` the binding `x = y;` would refer to the variable `y` in the surrounding scope, if one exists, and would be invalid if no such variable exists. That is, in a normal (non-recursive) set, attributes are not added to the lexical scope; in a recursive set, they are.

Recursive sets of course introduce the danger of infinite recursion. For example, the expression

    rec {
      x = y;
      y = x;
    }.x

will crash with an `infinite recursion encountered` error message.

## Let-expressions

A let-expression allows you to define local variables for an expression.

> let-in = `let` [ identifier = expr ]... `in` expr

Example:

    let
      x = "foo";
      y = "bar";
    in x + y

This evaluates to `"foobar"`.

## Inheriting attributes

When defining an attribute set or in a let-expression it is often convenient to copy variables from the surrounding lexical scope (e.g., when you want to propagate attributes). This can be shortened using the `inherit` keyword.

Example:

    let x = 123; in
    {
      inherit x;
      y = 456;
    }

is equivalent to

    let x = 123; in
    {
      x = x;
      y = 456;
    }

and both evaluate to `{ x = 123; y = 456; }`.

> Note

> This works because `x` is added to the lexical scope by the `let` construct.

It is also possible to inherit attributes from another attribute set.

Example:

In this fragment from `all-packages.nix`,

    graphviz = (import ../tools/graphics/graphviz) {
      inherit fetchurl stdenv libpng libjpeg expat x11 yacc;
      inherit (xorg) libXaw;
    };

    xorg = {
      libX11 = ...;
      libXaw = ...;
      ...
    }

    libpng = ...;
    libjpg = ...;
    ...

the set used in the function call to the function defined in `../tools/graphics/graphviz` inherits a number of variables from the surrounding scope (`fetchurl` ... `yacc`), but also inherits `libXaw` (the X Athena Widgets) from the `xorg` set.

Summarizing the fragment

    ...
    inherit x y z;
    inherit (src-set) a b c;
    ...

is equivalent to

    ...
    x = x; y = y; z = z;
    a = src-set.a; b = src-set.b; c = src-set.c;
    ...

when used while defining local variables in a let-expression or while defining a set.

## Functions

Functions have the following form:

    pattern: body

The pattern specifies what the argument of the function must look like, and binds variables in the body to (parts of) the argument. There are three kinds of patterns:

- If a pattern is a single identifier, then the function matches any argument. Example:

      let negate = x: !x;
          concat = x: y: x + y;
      in if negate true then concat "foo" "bar" else ""

  Note that `concat` is a function that takes one argument and returns a function that takes another argument. This allows partial parameterisation (i.e., only filling some of the arguments of a function); e.g.,

      map (concat "foo") [ "bar" "bla" "abc" ]

  evaluates to `[ "foobar" "foobla" "fooabc" ]`.

- A set pattern of the form `{ name1, name2, …, nameN }` matches a set containing the listed attributes, and binds the values of those attributes to variables in the function body. For example, the function

      { x, y, z }: z + y + x

  can only be called with a set containing exactly the attributes `x`, `y` and `z`. No other attributes are allowed. If you want to allow additional arguments, you can use an ellipsis (`...`):

      { x, y, z, ... }: z + y + x

  This works on any set that contains at least the three named attributes.

  It is possible to provide default values for attributes, in which case they are allowed to be missing. A default value is specified by writing `name ? e`, where e is an arbitrary expression. For example,

      { x, y ? "foo", z ? "bar" }: z + y + x

  specifies a function that only requires an attribute named `x`, but optionally accepts `y` and `z`.

- An `@`-pattern provides a means of referring to the whole value being matched:

      args@{ x, y, z, ... }: z + y + x + args.a

  but can also be written as:

      { x, y, z, ... } @ args: z + y + x + args.a

  Here `args` is bound to the argument as passed, which is further matched against the pattern `{ x, y, z, ... }`. The `@`-pattern makes mainly sense with an ellipsis(`...`) as you can access attribute names as `a`, using `args.a`, which was given as an additional attribute to the function.

  > Warning

  > `args@` binds the name `args` to the attribute set that is passed to the function. In particular, `args` does not include any default values specified with `?` in the function's set pattern.

  > For instance

  >     let
  >       f = args@{ a ? 23, ... }: [ a args ];
  >     in
  >       f {}

  > is equivalent to

  >     let
  >       f = args @ { ... }: [ (args.a or 23) args ];
  >     in
  >       f {}

  > and both expressions will evaluate to:

  >     [ 23 {} ]

Note that functions do not have names. If you want to give them a name, you can bind them to an attribute, e.g.,

    let concat = { x, y }: x + y;
    in concat { x = "foo"; y = "bar"; }

## Conditionals

Conditionals look like this:

    if e1 then e2 else e3

where e1 is an expression that should evaluate to a Boolean value (`true` or `false`).

## Assertions

Assertions are generally used to check that certain requirements on or between features and dependencies hold. They look like this:

    assert e1; e2

where e1 is an expression that should evaluate to a Boolean value. If it evaluates to `true`, e2 is returned; otherwise expression evaluation is aborted and a backtrace is printed.

Here is a Nix expression for the Subversion package that shows how assertions can be used:.

    { localServer ? false
    , httpServer ? false
    , sslSupport ? false
    , pythonBindings ? false
    , javaSwigBindings ? false
    , javahlBindings ? false
    , stdenv, fetchurl
    , openssl ? null, httpd ? null, db4 ? null, expat, swig ? null, j2sdk ? null
    }:

    assert localServer -> db4 != null; ①
    assert httpServer -> httpd != null && httpd.expat == expat; ②
    assert sslSupport -> openssl != null && (httpServer -> httpd.openssl == openssl); ③
    assert pythonBindings -> swig != null && swig.pythonSupport;
    assert javaSwigBindings -> swig != null && swig.javaSupport;
    assert javahlBindings -> j2sdk != null;

    stdenv.mkDerivation {
      name = "subversion-1.1.1";
      ...
      openssl = if sslSupport then openssl else null; ④
      ...
    }

The points of interest are:

1. This assertion states that if Subversion is to have support for local repositories, then Berkeley DB is needed. So if the Subversion function is called with the `localServer` argument set to `true` but the `db4` argument set to `null`, then the evaluation fails.

   Note that `->` is the logical implication Boolean operation.

2. This is a more subtle condition: if Subversion is built with Apache (`httpServer`) support, then the Expat library (an XML library) used by Subversion should be same as the one used by Apache. This is because in this configuration Subversion code ends up being linked with Apache code, and if the Expat libraries do not match, a build- or runtime link error or incompatibility might occur.

3. This assertion says that in order for Subversion to have SSL support (so that it can access `https` URLs), an OpenSSL library must be passed. Additionally, it says that if Apache support is enabled, then Apache's OpenSSL should match Subversion's. (Note that if Apache support is not enabled, we don't care about Apache's OpenSSL.)

4. The conditional here is not really related to assertions, but is worth pointing out: it ensures that if SSL support is disabled, then the Subversion derivation is not dependent on OpenSSL, even if a non-`null` value was passed. This prevents an unnecessary rebuild of Subversion if OpenSSL changes.

## With-expressions

A with-expression,

    with e1; e2

introduces the set e1 into the lexical scope of the expression e2. For instance,

    let as = { x = "foo"; y = "bar"; };
    in with as; x + y

evaluates to `"foobar"` since the `with` adds the `x` and `y` attributes of `as` to the lexical scope in the expression `x + y`. The most common use of `with` is in conjunction with the `import` function. E.g.,

    with (import ./definitions.nix); ...

makes all attributes defined in the file `definitions.nix` available as if they were defined locally in a `let`-expression.

The bindings introduced by `with` do not shadow bindings introduced by other means, e.g.

    let a = 3; in with { a = 1; }; let a = 4; in with { a = 2; }; ...

establishes the same scope as

    let a = 1; in let a = 2; in let a = 3; in let a = 4; in ...

## Comments

Comments can be single-line, started with a `#` character, or inline/multi-line, enclosed within `/* ... */`.

## Context-dependent keywords

`__curPos`

  A quasi-constant which will be replaced with an attribute set describing the location where `__curPos` was used, with attributes `file`, `line`, and `column`. For example, `import ./file.nix` will result in

      {
        column = 1;
        file = "/path/to/some/file.nix";
        line = 1;
      }

  assuming `file.nix` contains nothing but `__curPos`.

  In context without a source file (such as `nix-repl`), it will always be replaced with `null`:

      nix-repl> __curPos
      null

  While it may vaguely look like a builtin, this is a very different beast that is handled directly by the parser. It thus cannot be shadowed, bound to a different name, and is also not available under `builtins`.

      nix-repl> let __curPos = "no"; in __curPos
      null

  Despite this `__curPos`, much like `or`, may still be used as an identifier, it is only treated specially when it appears as an unqualified name:

      nix-repl> { __curPos = 1; }.__curPos
      1

`or`

  `or` is used in Attribute selection, where it is a keyword.

  However, it is not a keyword in some other contexts, and can be used as a binding name in attribute sets, let-bindings, non-initial function application position, and as a label in attribute paths.

  Its use as anything other than a keyword is discouraged.