Add 'nix eval' manpage

src/nix/eval.cc

@@ -40,30 +40,11 @@ struct CmdEval : MixJSON, InstallableCommand
         return "evaluate a Nix expression";
     }
 
-    Examples examples() override
+    std::string doc() override
     {
-        return {
-            {
-                "To evaluate a Nix expression given on the command line:",
-                "nix eval --expr '1 + 2'"
-            },
-            {
-                "To evaluate a Nix expression from a file or URI:",
-                "nix eval -f ./my-nixpkgs hello.name"
-            },
-            {
-                "To get the current version of Nixpkgs:",
-                "nix eval --raw nixpkgs#lib.version"
-            },
-            {
-                "To print the store path of the Hello package:",
-                "nix eval --raw nixpkgs#hello"
-            },
-            {
-                "To get a list of checks in the 'nix' flake:",
-                "nix eval nix#checks.x86_64-linux --apply builtins.attrNames"
-            },
-        };
+        return
+          #include "eval.md"
+          ;
     }
 
     Category category() override { return catSecondary; }

src/nix/eval.md

@@ -0,0 +1,74 @@
+R""(
+
+# Examples
+
+* Evaluate a Nix expression given on the command line:
+
+  ```console
+  # nix eval --expr '1 + 2'
+  ```
+
+* Evaluate a Nix expression to JSON:
+
+  ```console
+  # nix eval --json --expr '{ x = 1; }'
+  {"x":1}
+  ```
+
+* Evaluate a Nix expression from a file:
+
+  ```console
+  # nix eval -f ./my-nixpkgs hello.name
+  ```
+
+* Get the current version of the `nixpkgs` flake:
+
+  ```console
+  # nix eval --raw nixpkgs#lib.version
+  ```
+
+* Print the store path of the Hello package:
+
+  ```console
+  # nix eval --raw nixpkgs#hello
+  ```
+
+* Get a list of checks in the `nix` flake:
+
+  ```console
+  # nix eval nix#checks.x86_64-linux --apply builtins.attrNames
+  ```
+
+* Generate a directory with the specified contents:
+
+  ```console
+  # nix eval --write-to ./out --expr '{ foo = "bar"; subdir.bla = "123"; }'
+  # cat ./out/foo
+  bar
+  # cat ./out/subdir/bla
+  123
+
+# Description
+
+This command evaluates the Nix expression *installable* and prints the
+result on standard output.
+
+# Output format
+
+`nix eval` can produce output in several formats:
+
+* By default, the evaluation result is printed as a Nix expression.
+
+* With `--json`, the evaluation result is printed in JSON format. Note
+  that this fails if the result contains values that are not
+  representable as JSON, such as functions.
+
+* With `--raw`, the evaluation result must be a string, which is
+  printed verbatim, without any quoting.
+
+* With `--write-to` *path*, the evaluation result must be a string or
+  a nested attribute set whose leaf values are strings. These strings
+  are written to files named *path*/*attrpath*. *path* must not
+  already exist.
+
+)""