Add lib.to_file mirroring builtins.toFile from Nix

Also add scaffolding for adding library tests to checks.

lib/contracts.ncl

@@ -10,6 +10,10 @@ let predicate | doc "Various predicates used to define contracts"
       std.is_record x
       && std.record.has_field type_field x
       && x."%{type_field}" == "nixPlaceholder",
+    is_nix_to_file = fun x =>
+      std.is_record x
+      && std.record.has_field type_field x
+      && x."%{type_field}" == "nixToFile",
     is_nix_input = fun x =>
       std.is_record x
       && std.record.has_field type_field x
@@ -36,6 +40,7 @@ let predicate | doc "Various predicates used to define contracts"
       || std.is_string x
       || is_nix_path x
       || is_nix_placeholder x
+      || is_nix_to_file x
   }
   in
 
@@ -305,6 +310,14 @@ from the Nix world) or a derivation defined in Nickel.
       output | String,
     },
 
+  NixToFile
+    | doc "A path to the given output resolved later in the Nix store"
+    = {
+      "%{type_field}" | force = "nixToFile",
+      name | String,
+      text | NixString,
+    },
+
   OrganistShells = {
     dev | NickelDerivation = build,
     build | NickelDerivation,

lib/lib.ncl

@@ -20,5 +20,7 @@ let contracts = import "contracts.ncl" in
   import_nix | contracts.NixInputSugar -> contracts.NixInput
     = fun x => x,
   placeholder | String -> contracts.NixPlaceholder
-    = fun _output => { output = _output }
+    = fun _output => { output = _output },
+  to_file | String -> contracts.NixString -> contracts.NixToFile
+    = fun _name _text => { name = _name, text = _text },
 }

lib/lib.nix

@@ -116,6 +116,8 @@
             lib.getAttrFromPath chosenAttrPath flakeInputs
           else if organistType == "nixPlaceholder"
           then builtins.placeholder value.output
+          else if organistType == "nixToFile"
+          then builtins.toFile value.name (importFromNickel_ value.text)
           else builtins.mapAttrs (_: importFromNickel_) value
       )
     else if (type == "list")

project.ncl

@@ -55,5 +55,7 @@ let import_nix = organist.lib.import_nix in
         "%,
       },
     },
+
+  flake.checks = import "tests/main.ncl",
 }
   | organist.contracts.OrganistExpression

tests/main.ncl

@@ -0,0 +1,3 @@
+{
+  to_file = import "to_file.ncl",
+}

tests/to_file.ncl

@@ -0,0 +1,12 @@
+let organist = import "../lib/nix.ncl" in
+let file1 = organist.lib.to_file "file1" "important data" in
+let file2 = organist.lib.to_file "file2" nix-s%"see %{file1}"% in
+organist.builders.NixpkgsPkg
+& {
+  name = "test-to_path",
+  env.buildCommand = nix-s%"
+    [[ $(cat %{file1}) == "important data" ]]
+    [[ $(cat %{file2}) == "see /nix/store/ypiiqm7ig0fzqfz3v4j05g54ffk8svg9-file1" ]]
+    echo OK > $out
+  "%,
+}