# Syntax Summary

Below is a summary of the most important syntactic constructs in the Nix expression language. It’s not complete. In particular, there are many other built-in functions. See the Nix manual for the rest.

Example | Description
--- | ---
Basic values
`"Hello world"` | A string
`"${pkgs.bash}/bin/sh"` | A string containing an expression (expands to `"/nix/store/hash-bash-version/bin/sh"`)
`true`, `false` | Booleans
`123` | An integer
`./foo.png` | A path (relative to the containing Nix expression)
Compound values
`{ x = 1; y = 2; }` | An set with attributes names `x` and `y`
`{ foo.bar = 1; }` | A nested set, equivalent to `{ foo = { bar = 1; }; }`
`rec { x = "foo"; y = x + "bar"; }` | A recursive set, equivalent to `{ x = "foo"; y = "foobar"; }`
`[ "foo" "bar" ]` | A list with two elements
Operators
`"foo" + "bar"` | String concatenation
`1 + 2` | Integer addition
`"foo" == "f" + "oo"` | Equality test (evaluates to `true`)
`"foo" != "bar"` | Inequality test (evaluates to `true`)
`!true` | Boolean negation
`{ x = 1; y = 2; }.x` | Attribute selection (evaluates to `1`)
`{ x = 1; y = 2; }.z or 3` | Attribute selection with default (evaluates to `3`)
`{ x = 1; y = 2; } // { z = 3; }` | Merge two sets (attributes in the right-hand set taking precedence)
Control structures
`if 1 + 1 == 2 then "yes!" else "no!"` | Conditional expression
`assert 1 + 1 == 2; "yes!"` | Assertion check (evaluates to `"yes!"`)
`let x = "foo"; y = "bar"; in x + y` | Variable definition
`with pkgs.lib; head [ 1 2 3 ]` | Add all attributes from the given set to the scope (evaluates to `1`)
Functions (lambdas)
`x: x + 1` | A function that expects an integer and returns it increased by 1
`(x: x + 1) 100` | A function call (evaluates to 101)
`let inc = x: x + 1; in inc (inc (inc 100))` | A function bound to a variable and subsequently called by name (evaluates to 103)
`{ x, y }: x + y` | A function that expects a set with required attributes `x` and `y` and concatenates them
`{ x, y ? "bar" }: x + y` | A function that expects a set with required attribute `x` and optional `y`, using `"bar"` as default value for `y`
`{ x, y, ... }: x + y` | A function that expects a set with required attributes `x` and `y` and ignores any other attributes
`{ x, y } @ args: x + y` | A function that expects a set with required attributes `x` and `y`, and binds the whole set to `args`
Built-in functions
`import ./foo.nix` | Load and return Nix expression in given file
`map (x: x + x) [ 1 2 3 ]` | Apply a function to every element of a list (evaluates to `[ 2 4 6 ]`)