docs/concept: new FAQ: list arithmetic ops removal

This adds an FAQ guide that explains the removal of list arithmetic
operators ("+" and "*") from CUE v0.11 and later. It requires the tip of
cmd/cue, upgraded in an earlier commit, in order to access a version of
`cue fix` that understands how to update the operators.

It also updates the /e/v0.11-list-arithmetic redirector's target to
point at the FAQ so that cmd/cue can succinctly link from error messages
introduced in https://cuelang.org/cl/1200221.

Closes cue-lang/docs-and-content#178

Preview-Path: /docs/concept/faq/removing-list-arithmetic-operators-v0.11/
Preview-Path: /e/v0.11-list-arithmetic
Signed-off-by: Jonathan Matthews <github@hello.jonathanmatthews.com>
Change-Id: I8d50c273ccedb7cf6ca5295917032d5aa822b413
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cuelang.org/+/1200357
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>

content/docs/concept/faq/removing-list-arithmetic-operators-v0.11/en.md

@@ -0,0 +1,196 @@
+---
+title: Removing list arithmetic operators in CUE 0.11
+authors: [jpluscplusm]
+tags: [language]
+toc_hide: true
+---
+
+{{{with _script_ "en" "HIDDEN: set up access to CUE v0.11 before release"}}}
+export PATH=/cues/$CUELANG_CUE_TIP:$PATH
+{{{end}}}
+
+This guide answers some questions about a language change that happened in CUE
+version 0.11 -- including how to tell if the change affects your CUE, and how
+to update any CUE that's affected.
+
+## What's changed, and how can I tell if my CUE is affected?
+
+From version 0.11 onwards, the list arithmetic operators `*` and `+` don't work
+in CUE evaluated by the `cue` command and the Go API.
+Your CUE is not affected if a 0.11 version of `cue vet` evaluates it successfully.
+
+This CUE no longer works, and must be updated as
+[demonstrated below]({{< relref "removing-list-arithmetic-operators-v0.11" >}}#what-changes-are-needed-in-my-cue):
+
+{{{with upload "en" "removed"}}}
+-- list-arithmetic.cue --
+A: [1, 2, 3] * 2         // Invalid CUE!
+B: [1, 2, 3] + [4, 5, 6] // Invalid CUE!
+{{{end}}}
+
+The `*` and `+` operators continue to work as expected in all other situations,
+such as numeric arithmetic and string composition.
+They are **only** affected by this change if they are used with a list value on
+their left or right side.
+
+CUE versions 0.11 and later report the following errors when evaluating CUE
+that contains list arithmetic operators:
+
+{{{with script "en" "removed"}}}
+! cue vet list-arithmetic.cue
+{{{end}}}
+
+## What changes are needed in my CUE?
+
+- Wherever the `*` <!--vim*--> operator is used with a list:
+  use [`list.Repeat`](/go/pkg/list#Repeat) instead.
+- Wherever the `+` operator is used to combine lists:
+  use either [`list.Concat`](/go/pkg/list#Concat) or a nested `for` loop instead.
+- Use [`cue fix`]({{< relref "/docs/reference/command/cue-help-fix" >}}) to try
+  and make the changes automatically.
+
+[`cue fix`]({{< relref "/docs/reference/command/cue-help-fix" >}})
+will make these changes for you in many cases, but only if you are using `cue`
+command version\
+`v0.11.0-alpha.1` or later. Here's how to use `cue fix`:
+
+Start with some CUE that requires changes because it uses list arithmetic operators:
+
+{{{with upload "en" "changes"}}}
+-- changes-required.cue --
+// "[1, 2, 3] * 2" is no longer valid CUE.
+A: [1, 2, 3] * 2
+
+// "[1, 2, 3] + [4, 5, 6]" is no longer valid CUE.
+B: [1, 2, 3] + [4, 5, 6]
+
+// "[1, 2, 3] + [4, 5, 6]" can also be rewritten using a nested for loop:
+B: [for i in [1, 2, 3] {i}, for i in [4, 5, 6] {i}]
+
+// The operators can still be used, unchanged, to produce number, string, and bytes types.
+C: 1 + 2*3.3
+D: 2*"O." + 3*"X."
+E: 2*'X.' + 3*'O.'
+{{{end}}}
+
+Use `cue fix` to update files, packages, or entire modules.
+The command is silent unless it fails to process its inputs:
+
+{{{with script "en" "changes"}}}
+cue fix changes-required.cue
+{{{end}}}
+
+`cue fix` updates its inputs in place, rewriting files on disk:
+
+{{{with _script_ "en" "HIDDEN: changes: move file aside"}}}
+mv changes-required.cue .changes-required.cue
+{{{end}}}
+{{{with upload "en" "changes: updated file"}}}
+-- changes-required.cue --
+import "list"
+
+// "[1, 2, 3] * 2" is no longer valid CUE.
+A: list.Repeat([1, 2, 3], 2)
+
+// "[1, 2, 3] + [4, 5, 6]" is no longer valid CUE.
+B: list.Concat([[1, 2, 3], [4, 5, 6]])
+
+// "[1, 2, 3] + [4, 5, 6]" can also be rewritten using a nested for loop:
+B: [for i in [1, 2, 3] {i}, for i in [4, 5, 6] {i}]
+
+// The operators can still be used, unchanged, to produce number, string, and bytes types.
+C: 1 + 2*3.3
+D: 2*"O." + 3*"X."
+E: 2*'X.' + 3*'O.'
+{{{end}}}
+{{{with _script_ "en" "HIDDEN: changes: diff uploaded file"}}}
+diff changes-required.cue .changes-required.cue
+{{{end}}}
+
+## Why do I still get evaluation failures after running `cue fix`?
+
+`cue fix` can only update CUE where the list arithmetic operators are used with
+a literal list value (e.g. `[1, 2, 3]`). The command can't detect when they
+are used with a list value via a reference -- you must update these uses
+manually, for the time being.
+
+The following CUE would not be updated by `cue fix`:
+
+{{{with upload "en" "references: broken"}}}
+-- references.cue --
+// This file contains invalid CUE.
+x: [1, 2, 3]
+y: [4, 5, 6]
+
+A: x * 2 // Invalid use of the "*" operator with a list reference.
+B: x + y // Invalid use of the "+" operator with two list references.
+{{{end}}}
+
+This file must be
+[manually updated]({{< relref "removing-list-arithmetic-operators-v0.11" >}}#what-changes-are-needed-in-my-cue)
+with the following CUE:
+
+{{{with upload "en" "references: fixed"}}}
+#force
+-- references.cue --
+import "list"
+
+x: [1, 2, 3]
+y: [4, 5, 6]
+
+A: list.Repeat(x, 2)
+B: list.Concat([x, y])
+{{{end}}}
+
+{{{with _script_ "en" "HIDDEN: references: cue vet"}}}
+cue vet references.cue
+{{{end}}}
+
+## Why are list arithmetic operators being removed?
+<!-- TODO(jcm): move to a separate page, cf https://cuelang.org/cl/1200357/comment/9a648a4e_2a2d2c90/ -->
+
+Early versions of the CUE language allowed lists to be used with the arithmetic
+operators "`+`" and "`*`". This feature was removed from the language specification
+[some years ago](https://review.gerrithub.io/plugins/gitiles/cue-lang/cue/+/172f0060cd405f30c5873b793e44300e1a3588cb%5E%21/),
+with the removal being completed in CUE version 0.11.
+
+The commit that removed them from the CUE language specification in 2021
+explained the project's rationale:
+
+```
+List operators are confusing [and unnecessary].
+
+For instance, is the result of [a, ...] + [b, ...] open or closed?
+What about [a] + [b, ...]?
+```
+
+## Why were list arithmetic operators supported until now?
+<!-- TODO(jcm): move to a separate page, cf https://cuelang.org/cl/1200357/comment/9a648a4e_2a2d2c90/ -->
+
+The CUE project takes its compatibility responsibilities to its users
+seriously. A capability that's made available in a released version of CUE (as
+list arithmetic operators were) shouldn't be removed the instant that it's
+deprecated, even though CUE is still pre-version-1.0 and such an immediate
+removal might seem generally allowable under
+[semantic versioning guidelines](https://semver.org/#spec-item-5).
+
+Users might have written CUE that relied on list arithmetic operators, so
+the CUE tooling supported this feature up to and including version 0.10 --
+which was released over 3½ years after the feature stopped being mentioned.
+
+This breaking language change isn't based on `language.version` from
+`cue.mod/module.cue` because a significant length of time has now passed
+without the feature being mentioned in the CUE language specification or
+official documentation.
+
+## I have a question that isn't answered here
+
+Please join the [CUE community]({{< relref "/community" >}}) on GitHub, Slack,
+or Discord and ask the question there -- we'll be happy to help!
+
+## Related content
+
+- {{<linkto/related/reference "command/cue-help-fix" >}}
+- {{<linkto/related/reference "command/cue-help-vet" >}}
+- Standard library: [`list.Repeat`](/go/pkg/list#Repeat)
+- Standard library: [`list.Concat`](/go/pkg/list#Concat)

content/docs/concept/faq/removing-list-arithmetic-operators-v0.11/gen_cache.cue

@@ -0,0 +1,65 @@
+package site
+{
+	content: {
+		docs: {
+			concept: {
+				faq: {
+					"removing-list-arithmetic-operators-v0.11": {
+						page: {
+							cache: {
+								upload: {
+									removed:                 "ZrYNh0aXMmTn5yX+27Kyd9rBh5Q/FP1/OsZB1+YwndU="
+									changes:                 "DQwz/GXKFZNswBJqhsWgwA7XLbKLq9NMtZqivktazYE="
+									"changes: updated file": "byZqfYmj37XFAvrSv3qXpF+JhUm8vRy2ycsIIUAlONc="
+									"references: broken":    "KrG7zQyCMalImCNHA9LiVYOiRb6Qn2GgxZRt5juhMOA="
+									"references: fixed":     "rgsOJByXSen1JSW67YKQTDKEhsM0H5oYzE+iYX2gK1A="
+								}
+								multi_step: {
+									hash:       "R46R7IN0R0R0SV3QMUC4MJ4G95C15LNPTPTM1VHRCCTMVDES33DG===="
+									scriptHash: "BFJRNTKGBNJD4CC4J0LQRPNHHIU24FAVCCJMB3DKELBVIUBH6QP0===="
+									steps: [{
+										doc:      ""
+										cmd:      "export PATH=/cues/v0.11.0-0.dev.0.20240904084811-bb24c7ce1f04:$PATH"
+										exitCode: 0
+										output:   ""
+									}, {
+										doc:      ""
+										cmd:      "cue vet list-arithmetic.cue"
+										exitCode: 1
+										output: """
+												A: Multiplication of lists is superseded by list.Repeat; see https://cuelang.org/e/v0.11-list-arithmetic:
+												    ./list-arithmetic.cue:1:4
+												B: Addition of lists is superseded by list.Concat; see https://cuelang.org/e/v0.11-list-arithmetic:
+												    ./list-arithmetic.cue:2:4
+
+												"""
+									}, {
+										doc:      ""
+										cmd:      "cue fix changes-required.cue"
+										exitCode: 0
+										output:   ""
+									}, {
+										doc:      ""
+										cmd:      "mv changes-required.cue .changes-required.cue"
+										exitCode: 0
+										output:   ""
+									}, {
+										doc:      ""
+										cmd:      "diff changes-required.cue .changes-required.cue"
+										exitCode: 0
+										output:   ""
+									}, {
+										doc:      ""
+										cmd:      "cue vet references.cue"
+										exitCode: 0
+										output:   ""
+									}]
+								}
+							}
+						}
+					}
+				}
+			}
+		}
+	}
+}

content/docs/concept/faq/removing-list-arithmetic-operators-v0.11/page.cue

@@ -0,0 +1,3 @@
+package site
+
+content: docs: concept: faq: "removing-list-arithmetic-operators-v0.11": page: _