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Note recommended use of 'or' in override patterns

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This fixes #6
This commit is contained in:
Vincent Ambo 2021-11-14 15:53:00 +03:00
parent 2a098e081b
commit a9a1a46458
1 changed files with 9 additions and 2 deletions
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11
nix/nix-1p/README.md
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@ -549,7 +549,7 @@ like this:
```nix
someProgram.overrideAttrs(old: {
configureFlags = old.configureFlags ++ ["--mimic-threaten-tag"];
configureFlags = old.configureFlags or [] ++ ["--mimic-threaten-tag"];
})
```
@ -557,6 +557,13 @@ This pattern has a variety of applications of varying complexity. The top-level
package set itself can have an `overlays` argument passed to it which may add
new packages to the imported set.
Note the use of the `or` operator to default to an empty list if the
original flags do not include `configureFlags`. This is required in
case a package does not set any flags by itself.
Since this can change in a package over time, it is useful to guard
against it using `or`.
For a slightly more advanced example, assume that we want to import `<nixpkgs>`
but have the modification above be reflected in the imported package set:
@ -564,7 +571,7 @@ but have the modification above be reflected in the imported package set:
let
overlay = (self: super: {
someProgram = super.someProgram.overrideAttrs(old: {
configureFlags = old.configureFlags ++ ["--mimic-threaten-tag"];
configureFlags = old.configureFlags or [] ++ ["--mimic-threaten-tag"];
});
});
in import <nixpkgs> { overlays = [ overlay ]; }