lib/generators: toPretty

`toPretty` implements a pretty printer for nix values.

lib/generators.nix

@@ -90,4 +90,36 @@ rec {
     * parsers as well.
     */
   toYAML = {}@args: toJSON args;
+
+  # TODO we need some kind of pattern matching sometimes
+  /* Pretty print a value, akin to `builtins.trace`.
+    * Should probably be a builtin as well.
+    */
+  toPretty = {
+    /* If this option is true, attrsets like { __pretty = fn; val = …; }
+       will use fn to convert val to a pretty printed representation.
+       (This means fn is type Val -> String.) */
+    allowPrettyValues ? false
+  }@args: v: with builtins;
+    if      isInt      v then toString v
+    else if isBool     v then (if v == true then "true" else "false")
+    else if isString   v then "\"" + v + "\""
+    else if null ==    v then "null"
+    else if isFunction v then "<λ>"
+    else if isList     v then "[ "
+        + libStr.concatMapStringsSep " " (toPretty args) v
+      + " ]"
+    else if isAttrs    v then
+      # apply pretty values if allowed
+      if attrNames v == [ "__pretty" "val" ] && allowPrettyValues
+         then v.__pretty v.val
+      # TODO: there is probably a better representation?
+      else if v ? type && v.type == "derivation" then "<δ>"
+      else "{ "
+          + libStr.concatStringsSep " " (libAttr.mapAttrsToList
+              (name: value:
+                "${toPretty args name} = ${toPretty args value};") v)
+        + " }"
+    else "toPretty: should never happen (v = ${v})";
+
 }

lib/tests/misc.nix

@@ -285,6 +285,36 @@ runTests {
       expected = builtins.toJSON val;
   };
 
+  testToPretty = {
+    expr = mapAttrs (const (generators.toPretty {})) rec {
+      int = 42;
+      bool = true;
+      string = "fnord";
+      null_ = null;
+      function = x: x;
+      list = [ 3 4 function [ false ] ];
+      attrs = { foo = null; "foo bar" = "baz"; };
+      drv = derivation { name = "test"; system = builtins.currentSystem; };
+    };
+    expected = rec {
+      int = "42";
+      bool = "true";
+      string = "\"fnord\"";
+      null_ = "null";
+      function = "<λ>";
+      list = "[ 3 4 ${function} [ false ] ]";
+      attrs = "{ \"foo\" = null; \"foo bar\" = \"baz\"; }";
+      drv = "<δ>";
+    };
+  };
+
+  testToPrettyAllowPrettyValues = {
+    expr = generators.toPretty { allowPrettyValues = true; }
+             { __pretty = v: "«" + v + "»"; val = "foo"; };
+    expected  = "«foo»";
+  };
+
+
 # MISC
 
   testOverridableDelayableArgsTest = {