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lib.trivial.warnIf: init

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It's a common pattern in Nixpkgs to want to emit a warning in certain
cases, but not actually change behaviours.

This is often expressed as either
	if cond then lib.warn "Don't do that thing" x else x
Or
	(if cond then lib.warn "Don't do that thing" else lib.id) x

Neither of which really expresses the intent here, because it looks
like 'x' is being chosen conditionally.

To make this clearer, I introduce a "warnIf" function, which makes it
clear that the only thing being affected by the condition is whether
the warning is generated, not the value being returned.
This commit is contained in:
Alyssa Ross 2021-04-27 10:56:51 +00:00
parent e57815f095
commit 81e1e68eaf
2 changed files with 6 additions and 2 deletions
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5
lib/default.nix
View file

@ -66,8 +66,9 @@ let
stringLength sub substring tail trace;
inherit (self.trivial) id const pipe concat or and bitAnd bitOr bitXor
bitNot boolToString mergeAttrs flip mapNullable inNixShell isFloat min max
importJSON importTOML warn info showWarnings nixpkgsVersion version mod compare
splitByAndCompare functionArgs setFunctionArgs isFunction toHexString toBaseDigits;
importJSON importTOML warn warnIf info showWarnings nixpkgsVersion version
mod compare splitByAndCompare functionArgs setFunctionArgs isFunction
toHexString toBaseDigits;
inherit (self.fixedPoints) fix fix' converge extends composeExtensions
composeManyExtensions makeExtensible makeExtensibleWithCustomName;
inherit (self.attrsets) attrByPath hasAttrByPath setAttrByPath

3
lib/trivial.nix
View file

@ -297,12 +297,15 @@ rec {
# Usage:
# {
# foo = lib.warn "foo is deprecated" oldFoo;
# bar = lib.warnIf (bar == "") "Empty bar is deprecated" bar;
# }
#
# TODO: figure out a clever way to integrate location information from
# something like __unsafeGetAttrPos.
warn = msg: builtins.trace "warning: ${msg}";
warnIf = cond: msg: if cond then warn msg else id;
info = msg: builtins.trace "INFO: ${msg}";
showWarnings = warnings: res: lib.fold (w: x: warn w x) res warnings;