lib.generators.toPretty

The following is the current implementation of this function.

toPretty =
    {
      allowPrettyValues ? false,
      multiline ? true,
      indent ? "",
    }:
    let
      go =
        indent: v:
        let
          introSpace = if multiline then "\n${indent}  " else " ";
          outroSpace = if multiline then "\n${indent}" else " ";
        in
        if isInt v then
          toString v
        # toString loses precision on floats, so we use toJSON instead. This isn't perfect
        # as the resulting string may not parse back as a float (e.g. 42, 1e-06), but for
        # pretty-printing purposes this is acceptable.
        else if isFloat v then
          builtins.toJSON v
        else if isString v then
          let
            lines = filter (v: !isList v) (split "\n" v);
            escapeSingleline = escape [
              "\\"
              "\""
              "\${"
            ];
            escapeMultiline = replaceStrings [ "\${" "''" ] [ "''\${" "'''" ];
            singlelineResult = "\"" + concatStringsSep "\\n" (map escapeSingleline lines) + "\"";
            multilineResult =
              let
                escapedLines = map escapeMultiline lines;
                # The last line gets a special treatment: if it's empty, '' is on its own line at the "outer"
                # indentation level. Otherwise, '' is appended to the last line.
                lastLine = last escapedLines;
              in
              "''"
              + introSpace
              + concatStringsSep introSpace (init escapedLines)
              + (if lastLine == "" then outroSpace else introSpace + lastLine)
              + "''";
          in
          if multiline && length lines > 1 then multilineResult else singlelineResult
        else if true == v then
          "true"
        else if false == v then
          "false"
        else if null == v then
          "null"
        else if isPath v then
          toString v
        else if isList v then
          if v == [ ] then
            "[ ]"
          else
            "[" + introSpace + concatMapStringsSep introSpace (go (indent + "  ")) v + outroSpace + "]"
        else if isFunction v then
          let
            fna = functionArgs v;
            showFnas = concatStringsSep ", " (
              mapAttrsToList (name: hasDefVal: if hasDefVal then name + "?" else name) fna
            );
          in
          if fna == { } then "<function>" else "<function, args: {${showFnas}}>"
        else if isAttrs v then
          # apply pretty values if allowed
          if allowPrettyValues && v ? __pretty && v ? val then
            v.__pretty v.val
          else if v == { } then
            "{ }"
          else if v ? type && v.type == "derivation" then
            "<derivation ${v.name or "???"}>"
          else
            "{"
            + introSpace
            + concatStringsSep introSpace (
              mapAttrsToList (
                name: value:
                "${escapeNixIdentifier name} = ${
                  addErrorContext "while evaluating an attribute `${name}`" (go (indent + "  ") value)
                };"
              ) v
            )
            + outroSpace
            + "}"
        else
          abort "generators.toPretty: should never happen (v = ${v})";
    in
    go indent;