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nix/doc/manual/rl-next/lambda-printing.md

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synopsis prs issues
Functions are printed with more detail 9606 7145

Functions and builtins are printed with more detail in nix repl, nix eval, builtins.trace, and most other places values are printed.

Before:

$ nix repl nixpkgs
nix-repl> builtins.map
«primop»

nix-repl> builtins.map lib.id
«primop-app»

nix-repl> builtins.trace lib.id "my-value"
trace: <LAMBDA>
"my-value"

$ nix eval --file functions.nix
{ id = <LAMBDA>; primop = <PRIMOP>; primop-app = <PRIMOP-APP>; }

After:

$ nix repl nixpkgs
nix-repl> builtins.map
«primop map»

nix-repl> builtins.map lib.id
«partially applied primop map»

nix-repl> builtins.trace lib.id "my-value"
trace: «lambda id @ /nix/store/8rrzq23h2zq7sv5l2vhw44kls5w0f654-source/lib/trivial.nix:26:5»
"my-value"

$ nix eval --file functions.nix
{ id = «lambda id @ /Users/wiggles/nix/functions.nix:2:8»; primop = «primop map»; primop-app = «partially applied primop map»; }

This was actually released in Nix 2.20, but wasn't added to the release notes so we're announcing it here. The historical release notes have been updated as well.