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Wrapping a value with `builtins.break` breaks most builtins #1165

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opened 2026-03-20 23:56:40 +00:00 by blokyk · 4 comments

blokyk commented

2026-03-20 23:56:40 +00:00

Describe the bug

Wrapping a value with builtins.break, which should be a transparent operation (like builtins.trace is), results in it being treated as a thunk by most builtins, instead of being seen as the underlying value.

Steps To Reproduce

Multiple examples:

nix-repl> builtins.isAttrs (builtins.break {})
false

nix-repl> builtins.mapAttrs (n: v: v) (break {})
error:
       … while calling the 'mapAttrs' builtin
         at «string»:1:1:
            1| builtins.mapAttrs (n: v: v) (break {})
             | ^

       … while evaluating the second argument passed to builtins.mapAttrs

       error: expected a set but found a thunk: «thunk»

nix-repl> map (x: x) (break [ 1 ])
error:
       … while calling the 'map' builtin
         at «string»:1:1:
            1| builtins.map (x: x) (break [ 1 ])
             | ^

       … while evaluating the second argument passed to builtins.map

       error: expected a list but found a thunk: «thunk»

Expected behavior

A call to builtins.break is completely transparent to its consumer, such that ∀f,e, f (break e) == f e. This is the way that, for example, builtins.trace and builtins.warn work (e.g. isAttrs (trace "foo" {}) == true).

`nix --version` output

nix (Lix, like Nix) 2.94.0
System type: x86_64-linux
Additional system types: i686-linux, x86_64-v1-linux, x86_64-v2-linux, x86_64-v3-linux, x86_64-v4-linux
Features: gc, signed-caches
System configuration file: /etc/nix/nix.conf
User configuration files: /home/blokyk/.config/nix/nix.conf:/etc/xdg/xdg-ubuntu-xorg/nix/nix.conf:/etc/xdg/nix/nix.conf
Store directory: /nix/store
State directory: /nix/var/nix
Data directory: /nix/store/vr4a39d4bw01793jl4qap839rlvmc143-lix-2.94.0/share

Additional context

From what i can tell, this is caused by prim_break not calling forceValue on its argument. Adding that call currently changes the output of tests repl_characterization/stack_vars and repl_characterization/regression_9917; this seems to be because it gives a different call stack:

(on the left, the final call stack with current nix; on the right, the final call stack with forceValue(args[0]) added in prim_break)

## Describe the bug Wrapping a value with `builtins.break`, which should be a transparent operation (like `builtins.trace` is), results in it being treated as a thunk by most builtins, instead of being seen as the underlying value. ## Steps To Reproduce Multiple examples: ```nix nix-repl> builtins.isAttrs (builtins.break {}) false nix-repl> builtins.mapAttrs (n: v: v) (break {}) error: … while calling the 'mapAttrs' builtin at «string»:1:1: 1| builtins.mapAttrs (n: v: v) (break {}) | ^ … while evaluating the second argument passed to builtins.mapAttrs error: expected a set but found a thunk: «thunk» nix-repl> map (x: x) (break [ 1 ]) error: … while calling the 'map' builtin at «string»:1:1: 1| builtins.map (x: x) (break [ 1 ]) | ^ … while evaluating the second argument passed to builtins.map error: expected a list but found a thunk: «thunk» ``` ## Expected behavior A call to `builtins.break` is completely transparent to its consumer, such that ∀f,e, `f (break e) == f e`. This is the way that, for example, `builtins.trace` and `builtins.warn` work (e.g. `isAttrs (trace "foo" {}) == true`). ## `nix --version` output ``` nix (Lix, like Nix) 2.94.0 System type: x86_64-linux Additional system types: i686-linux, x86_64-v1-linux, x86_64-v2-linux, x86_64-v3-linux, x86_64-v4-linux Features: gc, signed-caches System configuration file: /etc/nix/nix.conf User configuration files: /home/blokyk/.config/nix/nix.conf:/etc/xdg/xdg-ubuntu-xorg/nix/nix.conf:/etc/xdg/nix/nix.conf Store directory: /nix/store State directory: /nix/var/nix Data directory: /nix/store/vr4a39d4bw01793jl4qap839rlvmc143-lix-2.94.0/share ``` ## Additional context From what i can tell, this is caused by [`prim_break`](https://git.lix.systems/lix-project/lix/src/branch/83bca23d4a/lix/libexpr/primops.cc#L647) not calling `forceValue` on its argument. Adding that call currently changes the output of tests [`repl_characterization/stack_vars`](https://git.lix.systems/lix-project/lix/src/commit/7d361f1a82/tests/functional/repl_characterization/data/stack_vars.test) and [`repl_characterization/regression_9917`](https://git.lix.systems/lix-project/lix/src/commit/8a8715af89/tests/functional/repl_characterization/data/regression_9917.test); this seems to be because it gives a different call stack: ![Screenshot From 2026-03-21 00-30-15](/attachments/bb0e4afa-3f1f-4ba9-ba49-a75de9fca74e) (on the left, the final call stack with current nix; on the right, the final call stack with `forceValue(args[0])` added in `prim_break`)

Screenshot From 2026-03-21 00-30-15.png

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blokyk added the

bug

label

2026-03-20 23:56:40 +00:00

pennae commented

2026-03-21 00:21:13 +00:00

Owner

yup, that's a bug. break returns its input unchanged instead of forcing it first, as would be required by our invariants. it's been like this forever

yup, that's a bug. `break` returns its input unchanged instead of forcing it first, as would be required by our invariants. it's been like this forever :dragnupsidedown:

blokyk commented

2026-03-21 00:22:45 +00:00

Author

Yeah, i thought this was known for a while but when i mentioned it to piegames on matrix they asked me to fill it here

pennae added the

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2026-03-21 00:54:43 +00:00

blokyk commented

2026-03-21 01:09:56 +00:00

Author

fyi so there's no duplicate work: i'm making a cl for this (again it's basically just one line), including a test to make sure we don't regress on this (though i'll probably only finish it tomorrow, it's kinda late here)

❤️ 1

lix-bot commented

2026-03-21 16:58:34 +00:00

Member

This issue was mentioned on Gerrit on the following CLs:

commit message in cl/5422 ("libexpr/primops: make break force its argument")

This issue was mentioned on Gerrit on the following CLs: * commit message in [cl/5422](https://gerrit.lix.systems/c/lix/+/5422) ("libexpr/primops: make break force its argument")