toPretty

lib.generators.toPretty

Docs pulled from | This Revision | about 1 hour ago

Pretty print a value, akin to builtins.trace.

Should probably be a builtin as well.

The pretty-printed string should be suitable for rendering default values in the NixOS manual. In particular, it should be as close to a valid Nix expression as possible.

Inputs

Structured function argument

allowPrettyValues: 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.)

multiline: If this option is true, the output is indented with newlines for attribute sets and lists

indent: Initial indentation level

Value

The value to be pretty printed

Noogle detected

Implementation

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;