Nix Language
The Nix language is designed for conveniently creating and composing derivations – precise descriptions of how contents of existing files are used to derive new files. It is:
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domain-specific
It comes with built-in functions to integrate with the Nix store, which manages files and performs the derivations declared in the Nix language.
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declarative
There is no notion of executing sequential steps. Dependencies between operations are established only through data.
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pure
Values cannot change during computation. Functions always produce the same output if their input does not change.
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functional
Functions are like any other value. Functions can be assigned to names, taken as arguments, or returned by functions.
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lazy
Values are only computed when they are needed.
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dynamically typed
Type errors are only detected when expressions are evaluated.
Overview
This is an incomplete overview of language features, by example.
Example | Description |
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Basic values |
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A string |
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A multi-line string. Strips common prefixed whitespace. Evaluates to |
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String interpolation (expands to |
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Booleans |
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Null value |
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An integer |
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A floating point number |
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An absolute path |
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A path relative to the file containing this Nix expression |
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A home path. Evaluates to the |
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Search path for Nix files. Value determined by |
Compound values |
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A set with attributes named |
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A nested set, equivalent to |
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A recursive set, equivalent to |
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Lists with three elements. |
Operators |
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String concatenation |
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Integer addition |
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Equality test (evaluates to |
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Inequality test (evaluates to |
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Boolean negation |
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Attribute selection (evaluates to |
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Attribute selection with default (evaluates to |
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Merge two sets (attributes in the right-hand set taking precedence) |
Control structures |
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Conditional expression |
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Assertion check (evaluates to |
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Variable definition |
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Add all attributes from the given set to the scope (evaluates to |
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Adds the variables to the current scope (attribute set or |
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Adds the attributes, from the attribute set in parentheses, to the current scope (attribute set or |
Functions (lambdas) |
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A function that expects an integer and returns it increased by 1 |
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Curried function, equivalent to |
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A function call (evaluates to 101) |
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A function bound to a variable and subsequently called by name (evaluates to 103) |
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A function that expects a set with required attributes |
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A function that expects a set with required attribute |
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A function that expects a set with required attributes |
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A function that expects a set with required attributes |
Built-in functions |
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Load and return Nix expression in given file |
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Apply a function to every element of a list (evaluates to |