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| -rw-r--r-- | nix/nix-1p/README.md | 34 |
1 files changed, 32 insertions, 2 deletions
diff --git a/nix/nix-1p/README.md b/nix/nix-1p/README.md index 72297a555127..5d2dce83bf20 100644 --- a/nix/nix-1p/README.md +++ b/nix/nix-1p/README.md @@ -19,6 +19,7 @@ and entering code snippets there. - [Language constructs](#language-constructs) - [Primitives / literals](#primitives--literals) - [Operators](#operators) + - [`//` (merge) operator](#-merge-operator) - [Variable bindings](#variable-bindings) - [Functions](#functions) - [Multiple arguments (currying)](#multiple-arguments-currying) @@ -113,8 +114,6 @@ rec { a = 15; b = a * 2; } Nix has several operators, most of which are unsurprising: -Make sure to understand the `//`-operator, as it is used quite a lot and is -probably the least familiar one. | Syntax | Description | |---------------------------|-----------------------------------------------------------------------------| | `+`, `-`, `*`, `/` | Numerical operations | @@ -131,6 +130,37 @@ probably the least familiar one. | `left // right` | Merge `left` & `right` attribute sets, with the right set taking precedence | +### `//` (merge) operator + +The `//`-operator is used pervasively in Nix code. You should familiarise +yourself with it, as it is likely also the least familiar one. + +It merges the left and right attribute sets given to it: + +```nix +{ a = 1; } // { b = 2; } + +# yields { a = 1; b = 2; } +``` + +Values from the right side take precedence: + +```nix +{ a = "left"; } // { a = "right"; } + +# yields { a = "right"; } +``` + +The merge operator does *not* recursively merge attribute sets; + +```nix +{ a = { b = 1; }; } // { a = { c = 2; }; } + +# yields { a = { c = 2; }; } +``` + +Helper functions for recursive merging exist in the [`lib` library](#pkgslib). + ## Variable bindings Bindings in Nix are introduced locally via `let` expressions, which make some |