In reply to @blokyk:matrix.org
sorry to interrupt the debate, but is the fact that break doesn't force its argument (unlike e.g. trace and warn) really intended? it causes break calls to be opaque in some situations (e.g. isAttrs {} != isAttrs (break {}), and mapAttrs f (break {}) always errors out, complaining about about the argument being a thunk and not a set), which means that it can potentially break (ha) a lot of code that relies on those. does that mean it'd be considered a breaking change to do that (is break even considered part of the language's stability contract?)
(and from experience it has bitten me every time i use break, to the point that i carry the declaration brk = x: builtins.seq (break x) x in every file i debug)

In reply to @blokyk:matrix.org
me too, but i don't know how the evaluator actually works internally, and i saw that trace has a forceValue(args[1]), and i know for a fact that trace doesn't have that bug, so i figured it'd fix it

nix-repl> builtins.mapAttrs (n: v: v) (break { a = 5; })
error:
       … while calling the 'mapAttrs' builtin
         at «string»:1:1:
            1| builtins.mapAttrs (n: v: v) (break { a = 5; })
             | ^

       … while evaluating the second argument passed to builtins.mapAttrs

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

nix-repl> builtins.mapAttrs (n: v: v) (builtins.trace "hi" { a = 5; })
trace: hi
{ a = 5; }

to me the most dangerous part is this, cause it's a lot more silent:

nix-repl> builtins.isAttrs (builtins.trace "hi" { a = 5; })            
trace: hi
true

nix-repl> builtins.isAttrs (break { a = 5; })               
false

another fun one is this :D

nix-repl> builtins.map (x: x) (break [])
[ ]

nix-repl> builtins.map (x: x) (break [ 5 ])
error:
       … while calling the 'map' builtin
         at «string»:1:1:
            1| builtins.map (x: x) (break [ 5 ])
             | ^

       … while evaluating the second argument passed to builtins.map

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

In reply to @blokyk:matrix.org
i wasn't sure how widely known this was, since i encountered it all the time (on both cppnix and lix)