Add a crude file generation mechanism

Running `nix run .\#regenerate-files` now generates files locally
according to the `files` option in `project.ncl`.

This is still rather crude in a number of ways, but it is already
working (and used to generate this project's own `.gitignore`).

Fixes #144

.gitignore

@@ -1,2 +1,2 @@
 /examples/*/result
-/result
+/result

lib/files.ncl

@@ -0,0 +1,84 @@
+let nix = import "./nix-interop/nix.ncl" in
+{
+  File = {
+    target
+      | doc m%"
+        The file to write to.
+        If null, defaults to the attribute name of the file.
+      "%
+      | String
+      | optional,
+    content
+      | doc m%"
+        The content of the file.
+      "%
+      | nix.derivation.NixString,
+    materialisation_method
+      : [| 'Symlink, 'Copy |]
+      | doc m%"
+        How the file should be materialized on-disk.
+
+        Symlinking makes it easier to track where the files are coming from,
+        but their target only exists after a first call to Organist, which
+        might be undesirable.
+      "%
+      | default
+      = 'Copy,
+    ..
+  },
+  Files = { _ : File },
+  NormaliseTargets = fun label files =>
+    files
+    |> std.record.map (fun name file_descr => file_descr & { target | default = name }),
+
+  Schema = {
+    files
+      | Files
+      | NormaliseTargets
+      | doc m%"
+        Set of files that should be generated in the project's directory.
+      "%
+      = {},
+    flake.apps.regenerate-files.program = nix-s%"%{regenerate_files files}/bin/regenerate-files"%,
+  },
+
+  regenerate_files | Files -> nix.derivation.Derivation = fun files_to_generate =>
+      let regnerate_one | String -> File -> nix.derivation.NixString = fun key file_descr =>
+          let file_content = file_descr.content in
+          let target = file_descr.target in
+          let copy_command =
+            match {
+              'Symlink => "ln -s",
+              'Copy => "cp",
+            }
+              file_descr.materialisation_method
+          in
+          let file_in_store =
+            nix.builtins.to_file
+              (nix.utils.escape_drv_name key)
+              file_content
+          in
+          nix-s%"
+            rm -f %{target}
+            echo "Regenerating %{target}"
+            %{copy_command} %{file_in_store} %{target}
+          "%
+        in
+      {
+        name = "regenerate-files",
+        content.text =
+          files_to_generate
+          |> std.record.to_array
+          |> std.array.map (fun { field, value } => regnerate_one field value)
+          |> std.array.fold_left
+            (
+              fun acc elt =>
+                nix-s%"
+                  %{acc}
+                  %{elt}
+                "%
+            )
+            "",
+      }
+        | nix.builders.ShellApplication,
+}

lib/nix-interop/nix.ncl

@@ -3,6 +3,7 @@
   builders = import "builders.ncl",
   shells = import "shells.ncl",
   builtins = import "builtins.ncl",
+  utils = import "utils.ncl",
 
   import_nix = builtins.import_nix,
 }

lib/nix-interop/utils.ncl

@@ -0,0 +1,12 @@
+{
+  escape_drv_name
+    : String -> String
+    | doc m%"
+      Escape the given string to make it an allowed derivation name
+    "%
+    = fun str =>
+      if std.string.is_match "^\\." str then
+        "-" ++ str
+      else
+        str
+}

lib/schema.ncl

@@ -1,4 +1,5 @@
 let nix = import "./nix-interop/nix.ncl" in
+let filegen = import "files.ncl" in
 {
   OrganistShells = {
     dev | nix.derivation.NickelDerivation = build,
@@ -20,11 +21,13 @@ let nix = import "./nix-interop/nix.ncl" in
   #
   # The contract must be: what the Nix side of the code can "parse" without
   # erroring out.
-  OrganistExpression = {
-    shells
-      | OrganistShells
-      | optional,
-    flake | FlakeOutputs | optional,
-    ..
-  },
+  OrganistExpression =
+    {
+      shells
+        | OrganistShells
+        | optional,
+      flake | FlakeOutputs = {},
+      ..
+    }
+    & filegen.Schema
 }

project.ncl

@@ -73,5 +73,13 @@ let import_nix = organist.nix.import_nix in
     },
 
   flake.checks = import "tests/main.ncl",
+
+  files.".gitignore".materialisation_method = 'Copy,
+  files.".gitignore".content = m%"
+    # This file is generated by Organist, please don't edit directly
+    /examples/*/result
+    /result
+
+  "%,
 }
   | organist.OrganistExpression