nix-super/doc/manual/rl-next/with-error-reporting.md

---
synopsis: Better error reporting for `with` expressions
prs: 9658
---

`with` expressions using non-attrset values to resolve variables are now reported with proper positions.

Previously an incorrect `with` expression would report no position at all, making it hard to determine where the error originated:

```
nix-repl> with 1; a
error:
       … <borked>

         at «none»:0: (source not available)

       error: value is an integer while a set was expected
```

Now position information is preserved and reported as with most other errors:

```
nix-repl> with 1; a
error:
       … while evaluating the first subexpression of a with expression
         at «string»:1:1:
            1| with 1; a
             | ^

       error: expected a set but found an integer
```
reduce the size of Env by one pointer since `up` and `values` are both pointer-aligned the type field will also be pointer-aligned, wasting 48 bits of space on most machines. we can get away with removing the type field altogether by encoding some information into the `with` expr that created the env to begin with, reducing the GC load for the absolutely massive amount of single-entry envs we create for lambdas. this reduces memory usage of system eval by quite a bit (reducing heap size of our system eval from 8.4GB to 8.23GB) and gives similar savings in eval time. running `nix eval --raw --impure --expr 'with import <nixpkgs/nixos> {}; system'` before: Time (mean ± σ): 5.576 s ± 0.003 s [User: 5.197 s, System: 0.378 s] Range (min … max): 5.572 s … 5.581 s 10 runs after: Time (mean ± σ): 5.408 s ± 0.002 s [User: 5.019 s, System: 0.388 s] Range (min … max): 5.405 s … 5.411 s 10 runs 2023-12-22 19:19:53 +02:00			`---`
			synopsis: Better error reporting for `with` expressions
			`prs: 9658`
			`---`

			`with` expressions using non-attrset values to resolve variables are now reported with proper positions.

			Previously an incorrect `with` expression would report no position at all, making it hard to determine where the error originated:

			```
Print value on type error Adds the failing value to `value is <TYPE> while a <TYPE> is expected` error messages. 2023-12-07 02:03:01 +02:00			`nix-repl> with 1; a`
reduce the size of Env by one pointer since `up` and `values` are both pointer-aligned the type field will also be pointer-aligned, wasting 48 bits of space on most machines. we can get away with removing the type field altogether by encoding some information into the `with` expr that created the env to begin with, reducing the GC load for the absolutely massive amount of single-entry envs we create for lambdas. this reduces memory usage of system eval by quite a bit (reducing heap size of our system eval from 8.4GB to 8.23GB) and gives similar savings in eval time. running `nix eval --raw --impure --expr 'with import <nixpkgs/nixos> {}; system'` before: Time (mean ± σ): 5.576 s ± 0.003 s [User: 5.197 s, System: 0.378 s] Range (min … max): 5.572 s … 5.581 s 10 runs after: Time (mean ± σ): 5.408 s ± 0.002 s [User: 5.019 s, System: 0.388 s] Range (min … max): 5.405 s … 5.411 s 10 runs 2023-12-22 19:19:53 +02:00			`error:`
			`… <borked>`

			`at «none»:0: (source not available)`

			`error: value is an integer while a set was expected`
			```

			`Now position information is preserved and reported as with most other errors:`

			```
			`nix-repl> with 1; a`
			`error:`
			`… while evaluating the first subexpression of a with expression`
			`at «string»:1:1:`
			`1\| with 1; a`
			`\| ^`

Print value on type error Adds the failing value to `value is <TYPE> while a <TYPE> is expected` error messages. 2023-12-07 02:03:01 +02:00			`error: expected a set but found an integer`
reduce the size of Env by one pointer since `up` and `values` are both pointer-aligned the type field will also be pointer-aligned, wasting 48 bits of space on most machines. we can get away with removing the type field altogether by encoding some information into the `with` expr that created the env to begin with, reducing the GC load for the absolutely massive amount of single-entry envs we create for lambdas. this reduces memory usage of system eval by quite a bit (reducing heap size of our system eval from 8.4GB to 8.23GB) and gives similar savings in eval time. running `nix eval --raw --impure --expr 'with import <nixpkgs/nixos> {}; system'` before: Time (mean ± σ): 5.576 s ± 0.003 s [User: 5.197 s, System: 0.378 s] Range (min … max): 5.572 s … 5.581 s 10 runs after: Time (mean ± σ): 5.408 s ± 0.002 s [User: 5.019 s, System: 0.388 s] Range (min … max): 5.405 s … 5.411 s 10 runs 2023-12-22 19:19:53 +02:00			```