cue: add documentation on writeCueValidator

doc/languages-frameworks/cuelang.section.md

@@ -0,0 +1,93 @@
+# Cue (Cuelang) {#cuelang}
+
+[Cuelang](https://cuelang.org/) is a language to:
+
+- describe schemas and validate backward-compatibility
+- generate code and schemas in various formats (e.g. JSON Schema, OpenAPI)
+- do configuration akin to [Dhall Lang](https://dhall-lang.org/)
+- perform data validation
+
+## Cuelang schema quick start
+
+Cuelang schemas are similar to JSON, here is a quick cheatsheet:
+
+- Default types includes: `null`, `string`, `bool`, `bytes`, `number`, `int`, `float`, lists as `[...T]` where `T` is a type.
+- All structures, defined by: `myStructName: { <fields> }` are **open** -- they accept fields which are not specified.
+- Closed structures can be built by doing `myStructName: close({ <fields> })` -- they are strict in what they accept.
+- `#X` are **definitions**, referenced definitions are **recursively closed**, i.e. all its children structures are **closed**.
+- `&` operator is the [unification operator](https://cuelang.org/docs/references/spec/#unification) (similar to a type-level merging operator), `|` is the [disjunction operator](https://cuelang.org/docs/references/spec/#disjunction) (similar to a type-level union operator).
+- Values **are** types, i.e. `myStruct: { a: 3 }` is a valid type definition that only allows `3` as value.
+
+- Read <https://cuelang.org/docs/concepts/logic/> to learn more about the semantics.
+- Read <https://cuelang.org/docs/references/spec/> to learn about the language specification.
+
+## `writeCueValidator`
+
+Nixpkgs provides a `pkgs.writeCueValidator` helper, which will write a validation script based on the provided Cuelang schema.
+
+Here is an example:
+```
+pkgs.writeCueValidator
+  (pkgs.writeText "schema.cue" ''
+    #Def1: {
+      field1: string
+    }
+  '')
+  { document = "#Def1"; }
+```
+
+- The first parameter is the Cue schema file.
+- The second paramter is an options parameter, currently, only: `document` can be passed.
+
+`document` : match your input data against this fragment of structure or definition, e.g. you may use the same schema file but differents documents based on the data you are validating.
+
+Another example, given the following `validator.nix` :
+```
+{ pkgs ? import <nixpkgs> {} }:
+let
+  genericValidator = version:
+  pkgs.writeCueValidator
+    (pkgs.writeText "schema.cue" ''
+      #Version1: {
+        field1: string
+      }
+      #Version2: #Version1 & {
+        field1: "unused"
+      }''
+    )
+    { document = "#Version${toString version}"; };
+in
+{
+  validateV1 = genericValidator 1;
+  validateV2 = genericValidator 2;
+}
+```
+
+The result is a script that will validate the file you pass as the first argument against the schema you provided `writeCueValidator`.
+
+It can be any format that `cue vet` supports, i.e. YAML or JSON for example.
+
+Here is an example, named `example.json`, given the following JSON:
+```
+{ "field1": "abc" }
+```
+
+You can run the result script (named `validate`) as the following:
+
+```console
+$ nix-build validator.nix
+$ ./result example.json
+$ ./result-2 example.json
+field1: conflicting values "unused" and "abc":
+    ./example.json:1:13
+    ../../../../../../nix/store/v64dzx3vr3glpk0cq4hzmh450lrwh6sg-schema.cue:5:11
+$ sed -i 's/"abc"/3/' example.json
+$ ./result example.json
+field1: conflicting values 3 and string (mismatched types int and string):
+    ./example.json:1:13
+    ../../../../../../nix/store/v64dzx3vr3glpk0cq4hzmh450lrwh6sg-schema.cue:5:11
+```
+
+**Known limitations**
+
+* The script will enforce **concrete** values and will not accept lossy transformations (strictness). You can add these options if you need them.

doc/languages-frameworks/index.xml

@@ -13,6 +13,7 @@
  <xi:include href="coq.section.xml" />
  <xi:include href="crystal.section.xml" />
  <xi:include href="cuda.section.xml" />
+ <xi:include href="cuelang.section.xml" />
  <xi:include href="dhall.section.xml" />
  <xi:include href="dotnet.section.xml" />
  <xi:include href="emscripten.section.xml" />

pkgs/development/tools/cue/validator.nix

@@ -1,4 +1,9 @@
 { cue, writeShellScript, lib }:
+# `document` must be a fragment of definition or structure that the input data will be matched against.
+# `document` must exist in the Cue schema file provided (`cueSchemaFile`).
+# The result is a script that can be used to validate the input data (JSON/YAML and more can be supported depending on Cue)
+# against the fragment described by `document` or the whole definition.
+# The script will be strict and enforce concrete values, i.e. partial documents are not supported.
 cueSchemaFile: { document ? null }:
   writeShellScript "validate-using-cue"
   ''${cue}/bin/cue \