lix/doc/manual/rl-next/deprecated-features.md

---
synopsis: Deprecated language features
issues: [fj#437, 861]
cls: [1785, 1736, 1735, 1744, 2206]
category: Breaking Changes
credits: [piegames, horrors]
---

A system for deprecation (and then the planned removal) of undesired language features has been put into place.
It is controlled via feature flags much like experimental features, except that the deprecations are enabled default,
and can be disabled via the flags for backwards compatibility (opt-out with `--extra-deprecated-features` or the Nix configuration file).

- `url-literals`: **URL literals** have long been obsolete and discouraged of use, and now they are officially deprecated.
  This means that all URLs must be properly put within quotes like all other strings.
- `rec-set-overrides`: **__overrides** is an old arcane syntax which has not been in use for more than a decade.
  It is soft-deprecated with a warning only, with the plan to turn that into an error in a future release.
- `ancient-let`: **The old `let` syntax** (`let { body = …; … }`) is soft-deprecated with a warning as well. Use the regular `let … in` instead.
- `shadow-internal-symbols`: Arithmetic expressions like `5 - 3` internally expand to `__sub 5 3`, where `__sub` maps to a subtraction builtin. Shadowing such a symbols would affect the evaluation of such operations, but in a very inconsistent way, and is therefore deprecated now. **Affected symbols are:** `__sub`, `__mul`, `__div`, `__lessThan`, `__findFile` and `__nixPath`. Note that these symbols may still be used as variable names as long as they do not shadow internal operations, so e.g. `let __sub = x: y: x + y; in __sub 3 5` remains valid code.
libexpr: Deprecate URL literals Closes #437. Change-Id: I9f67fc965bb4a7e7fd849e5067ac1cb3bab064cd 2024-07-13 05:55:17 +00:00			`---`
libexpr: Soft-deprecate __overrides Change-Id: I787e69e1dad6edc5ccdb747b74a9ccd6e8e13bb3 2024-08-18 14:02:23 +00:00			`synopsis: Deprecated language features`
libexpr: Deprecate overriding __sub and the like It was never intended to be a feature to be used, and moreover it is inconsistent: One cannot override `+`, and overriding `__lessThan` won't affect the builtins which do comparisons. Change-Id: Iaba54a05aa4c2eb37cdb3dc0d731fcee5a86deba 2024-11-18 11:56:46 +00:00			`issues: [fj#437, 861]`
			`cls: [1785, 1736, 1735, 1744, 2206]`
libexpr: Deprecate URL literals Closes #437. Change-Id: I9f67fc965bb4a7e7fd849e5067ac1cb3bab064cd 2024-07-13 05:55:17 +00:00			`category: Breaking Changes`
			`credits: [piegames, horrors]`
			`---`

libexpr: Soft-deprecate __overrides Change-Id: I787e69e1dad6edc5ccdb747b74a9ccd6e8e13bb3 2024-08-18 14:02:23 +00:00			`A system for deprecation (and then the planned removal) of undesired language features has been put into place.`
			`It is controlled via feature flags much like experimental features, except that the deprecations are enabled default,`
			and can be disabled via the flags for backwards compatibility (opt-out with `--extra-deprecated-features` or the Nix configuration file).
libexpr: Deprecate URL literals Closes #437. Change-Id: I9f67fc965bb4a7e7fd849e5067ac1cb3bab064cd 2024-07-13 05:55:17 +00:00
libexpr: Soft-deprecate __overrides Change-Id: I787e69e1dad6edc5ccdb747b74a9ccd6e8e13bb3 2024-08-18 14:02:23 +00:00			- `url-literals`: URL literals have long been obsolete and discouraged of use, and now they are officially deprecated.
			`This means that all URLs must be properly put within quotes like all other strings.`
			- `rec-set-overrides`: __overrides is an old arcane syntax which has not been in use for more than a decade.
			`It is soft-deprecated with a warning only, with the plan to turn that into an error in a future release.`
libexpr: Soft-deprecate ancient let syntax Change-Id: I6802b26f038578870ea1fa1ed298f0c4b1f29c4a 2024-08-18 15:28:37 +00:00			- `ancient-let`: The old `let` syntax (`let { body = …; … }`) is soft-deprecated with a warning as well. Use the regular `let … in` instead.
libexpr: Deprecate overriding __sub and the like It was never intended to be a feature to be used, and moreover it is inconsistent: One cannot override `+`, and overriding `__lessThan` won't affect the builtins which do comparisons. Change-Id: Iaba54a05aa4c2eb37cdb3dc0d731fcee5a86deba 2024-11-18 11:56:46 +00:00			- `shadow-internal-symbols`: Arithmetic expressions like `5 - 3` internally expand to `__sub 5 3`, where `__sub` maps to a subtraction builtin. Shadowing such a symbols would affect the evaluation of such operations, but in a very inconsistent way, and is therefore deprecated now. Affected symbols are: `__sub`, `__mul`, `__div`, `__lessThan`, `__findFile` and `__nixPath`. Note that these symbols may still be used as variable names as long as they do not shadow internal operations, so e.g. `let __sub = x: y: x + y; in __sub 3 5` remains valid code.