diff options
| -rw-r--r-- | nix/nix-1p/README.md | 11 |
1 files changed, 9 insertions, 2 deletions
diff --git a/nix/nix-1p/README.md b/nix/nix-1p/README.md index 6cc37634ed..bdb49c7def 100644 --- a/nix/nix-1p/README.md +++ b/nix/nix-1p/README.md @@ -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 ]; } |