lib/generators.toPretty: Improved string printing, handling newlines

lib/generators.nix

@@ -213,7 +213,19 @@ rec {
             outroSpace = if multiline then "\n${indent}" else " ";
     in if   isInt      v then toString v
     else if isFloat    v then "~${toString v}"
-    else if isString   v then ''"${libStr.escape [''"''] v}"''
+    else if isString   v then
+      let
+        # Separate a string into its lines
+        newlineSplits = filter (v: ! isList v) (builtins.split "\n" v);
+        # For a '' string terminated by a \n, which happens when the closing '' is on a new line
+        multilineResult = "''" + introSpace + concatStringsSep introSpace (lib.init newlineSplits) + outroSpace + "''";
+        # For a '' string not terminated by a \n, which happens when the closing '' is not on a new line
+        multilineResult' = "''" + introSpace + concatStringsSep introSpace newlineSplits + "''";
+        # For single lines, replace all newlines with their escaped representation
+        singlelineResult = "\"" + libStr.escape [ "\"" ] (concatStringsSep "\\n" newlineSplits) + "\"";
+      in if multiline && length newlineSplits > 1 then
+        if lib.last newlineSplits == "" then multilineResult else multilineResult'
+      else singlelineResult
     else if true  ==   v then "true"
     else if false ==   v then "false"
     else if null  ==   v then "null"

lib/tests/misc.nix

@@ -450,7 +450,9 @@ runTests {
       int = 42;
       float = 0.1337;
       bool = true;
+      emptystring = "";
       string = ''fno"rd'';
+      newlinestring = "\n";
       path = /. + "/foo";
       null_ = null;
       function = x: x;
@@ -463,7 +465,9 @@ runTests {
       int = "42";
       float = "~0.133700";
       bool = "true";
+      emptystring = ''""'';
       string = ''"fno\"rd"'';
+      newlinestring = "\"\\n\"";
       path = "/foo";
       null_ = "null";
       function = "<λ>";
@@ -478,6 +482,16 @@ runTests {
     expr = mapAttrs (const (generators.toPretty { })) rec {
       list = [ 3 4 [ false ] ];
       attrs = { foo = null; bar.foo = "baz"; };
+      newlinestring = "\n";
+      multilinestring = ''
+        hello
+        there
+        test
+      '';
+      multilinestring' = ''
+        hello
+        there
+        test'';
     };
     expected = rec {
       list = ''
@@ -495,6 +509,19 @@ runTests {
           };
           foo = null;
         }'';
+      newlinestring = "''\n  \n''";
+      multilinestring = ''
+        '''
+          hello
+          there
+          test
+        ''''';
+      multilinestring' = ''
+        '''
+          hello
+          there
+          test''''';
+
     };
   };