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Literals

Overview

Nix expressions are built from values. Literals are the syntax for writing primitive values directly in source code: integers, floats, booleans, null, and paths. Lists, attribute sets, and functions use their own forms and are covered elsewhere.

Strings have a richer literal surface (quotes, indentation, interpolation, antiquotation); see strings and interpolation. Paths are a distinct type from strings and interact with the filesystem and store paths when coerced or copied into the store.

Details

Integers are signed 64-bit two's complement values (range (-2^{63}) through (2^{63}-1)). Non-negative integers appear as digit sequences (0, 42). Negative values are unary - applied to a positive literal (-1), not a dedicated negative token. The minimum value cannot be written as a literal: -9223372036854775808 parses as negation of 9223372036854775808, which is one past the maximum and overflows.

Floats are 64-bit IEEE 754 numbers. Examples include 123.43 and scientific notation such as .27e13. A leading - is again unary negation.

Booleans are the keywords true and false. The same values exist as builtins.true and builtins.false.

Null is the single value written null, also available as builtins.null.

Paths denote filesystem locations and are not strings. A path literal must contain at least one /: ./src, /etc/nixos, or ~/projects. Without a slash, a dotted name is attribute selection—builder.sh selects sh from builder, not a path. Relative path literals resolve against the directory of the file being evaluated. Absolute paths start with /. The ~/ home shorthand is rejected in pure evaluation. Lookup paths such as <nixpkgs> also produce path values when resolved. Path literals may include interpolation, but at least one / must appear before any ${…} so the token is recognized as a path.

Unquoted tokens that match the URI grammar (http://example.com) are parsed as string literals, not paths—a convenience for URLs without quotes. See strings and interpolation.

Compound values—lists and attribute sets—use bracket and brace syntax rather than single-token literals. For how literals relate to the type system and coercion, see types and coercion.

Examples

# integers and floats
42
-7
3.14
.27e13

# booleans and null
true
false
null

# paths (must contain /)
./default.nix
../modules
/etc/hosts
~/src          # rejected under pure evaluation

# not a path: attribute selection
# builder.sh

# URI-like token → string, not path
https://nixos.org

# lookup path (resolves to a path when evaluated)
<nixpkgs>

Verified with nix-instantiate --eval: builtins.typeOf https://nixos.org → "string"; builtins.isPath ./. → true; ~/x under --pure-eval fails with a pure-mode path error.

References

See also