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

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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.