From b502b6682b9c73ae5760967eaf7161b3e8523e9e Mon Sep 17 00:00:00 2001 From: Maximilian Bosch Date: Mon, 20 May 2019 19:17:18 +0200 Subject: doc: clarify that optional attrs in a function argument will be ignored unless specified In `args@{ a ? 1 }: /* ... */` the value `a` won't be a part of `args` unless it's specified when calling the function, the default value will be ignored in this case. My personal point of view is that this behavior is a matter of taste, at least I was pretty sure that unmatched arguments will be a part of `args@` while debugging some Nix code last week. I decided to add a warning to the docs which hopefully reduces the confusion of further Nix developers who thought the same about `args@`. --- doc/manual/expressions/language-constructs.xml | 20 +++++++++++++++++++- 1 file changed, 19 insertions(+), 1 deletion(-) (limited to 'doc/manual') diff --git a/doc/manual/expressions/language-constructs.xml b/doc/manual/expressions/language-constructs.xml index f961ed921..923b5d8c4 100644 --- a/doc/manual/expressions/language-constructs.xml +++ b/doc/manual/expressions/language-constructs.xml @@ -217,7 +217,25 @@ but can also be written as: ellipsis(...) as you can access attribute names as a, using args.a, which was given as an additional attribute to the function. - + + + + + The args@ expression is bound to the argument passed to the function which + means that attributes with defaults that aren't explicitly specified in the function call + won't cause an evaluation error, but won't exist in args. + + + For instance + +let + function = args@{ a ? 23, ... }: args; +in + function {} + + will evaluate to an empty attribute set. + + -- cgit v1.2.3