nixpkgs/lib/generators.nix

/* Functions that generate widespread file
 * formats from nix data structures.
 *
 * They all follow a similar interface:
 * generator { config-attrs } data
 *
 * `config-attrs` are “holes” in the generators
 * with sensible default implementations that
 * can be overwritten. The default implementations
 * are mostly generators themselves, called with
 * their respective default values; they can be reused.
 *
 * Tests can be found in ./tests.nix
 * Documentation in the manual, #sec-generators
 */
{ lib }:
with (lib).trivial;
let
  libStr = lib.strings;
  libAttr = lib.attrsets;

  flipMapAttrs = flip libAttr.mapAttrs;

  inherit (lib) isFunction;
in

rec {

  ## -- HELPER FUNCTIONS & DEFAULTS --

  /* Convert a value to a sensible default string representation.
   * The builtin `toString` function has some strange defaults,
   * suitable for bash scripts but not much else.
   */
  mkValueStringDefault = {}: v: with builtins;
    let err = t: v: abort
          ("generators.mkValueStringDefault: " +
           "${t} not supported: ${toPretty {} v}");
    in   if isInt      v then toString v
    # we default to not quoting strings
    else if isString   v then v
    # isString returns "1", which is not a good default
    else if true  ==   v then "true"
    # here it returns to "", which is even less of a good default
    else if false ==   v then "false"
    else if null  ==   v then "null"
    # if you have lists you probably want to replace this
    else if isList     v then err "lists" v
    # same as for lists, might want to replace
    else if isAttrs    v then err "attrsets" v
    else if isFunction v then err "functions" v
    else err "this value is" (toString v);


  /* Generate a line of key k and value v, separated by
   * character sep. If sep appears in k, it is escaped.
   * Helper for synaxes with different separators.
   *
   * mkValueString specifies how values should be formatted.
   *
   * mkKeyValueDefault {} ":" "f:oo" "bar"
   * > "f\:oo:bar"
   */
  mkKeyValueDefault = {
    mkValueString ? mkValueStringDefault {}
  }: sep: k: v:
    "${libStr.escape [sep] k}${sep}${mkValueString v}";


  ## -- FILE FORMAT GENERATORS --


  /* Generate a key-value-style config file from an attrset.
   *
   * mkKeyValue is the same as in toINI.
   */
  toKeyValue = {
    mkKeyValue ? mkKeyValueDefault {} "="
  }: attrs:
    let mkLine = k: v: mkKeyValue k v + "\n";
    in libStr.concatStrings (libAttr.mapAttrsToList mkLine attrs);


  /* Generate an INI-style config file from an
   * attrset of sections to an attrset of key-value pairs.
   *
   * generators.toINI {} {
   *   foo = { hi = "${pkgs.hello}"; ciao = "bar"; };
   *   baz = { "also, integers" = 42; };
   * }
   *
   *> [baz]
   *> also, integers=42
   *>
   *> [foo]
   *> ciao=bar
   *> hi=/nix/store/y93qql1p5ggfnaqjjqhxcw0vqw95rlz0-hello-2.10
   *
   * The mk* configuration attributes can generically change
   * the way sections and key-value strings are generated.
   *
   * For more examples see the test cases in ./tests.nix.
   */
  toINI = {
    # apply transformations (e.g. escapes) to section names
    mkSectionName ? (name: libStr.escape [ "[" "]" ] name),
    # format a setting line from key and value
    mkKeyValue    ? mkKeyValueDefault {} "="
  }: attrsOfAttrs:
    let
        # map function to string for each key val
        mapAttrsToStringsSep = sep: mapFn: attrs:
          libStr.concatStringsSep sep
            (libAttr.mapAttrsToList mapFn attrs);
        mkSection = sectName: sectValues: ''
          [${mkSectionName sectName}]
        '' + toKeyValue { inherit mkKeyValue; } sectValues;
    in
      # map input to ini sections
      mapAttrsToStringsSep "\n" mkSection attrsOfAttrs;


  /* Generates JSON from an arbitrary (non-function) value.
    * For more information see the documentation of the builtin.
    */
  toJSON = {}: builtins.toJSON;


  /* YAML has been a strict superset of JSON since 1.2, so we
    * use toJSON. Before it only had a few differences referring
    * to implicit typing rules, so it should work with older
    * parsers as well.
    */
  toYAML = {}@args: toJSON args;


  /* 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 isString   v then ''"${libStr.escape [''"''] v}"''
    else if true  ==   v then "true"
    else if false ==   v then "false"
    else if null ==    v then "null"
    else if isFunction v then
      let fna = lib.functionArgs v;
          showFnas = concatStringsSep "," (libAttr.mapAttrsToList
                       (name: hasDefVal: if hasDefVal then "(${name})" else name)
                       fna);
      in if fna == {}    then "<λ>"
                         else "<λ:{${showFnas}}>"
    else if isList     v then "[ "
        + libStr.concatMapStringsSep " " (toPretty args) v
      + " ]"
    else if isAttrs    v then
      # apply pretty values if allowed
      if attrNames v == [ "__pretty" "val" ] && allowPrettyValues
         then v.__pretty v.val
      # TODO: there is probably a better representation?
      else if v ? type && v.type == "derivation" then "<δ>"
      else "{ "
          + libStr.concatStringsSep " " (libAttr.mapAttrsToList
              (name: value:
                "${toPretty args name} = ${toPretty args value};") v)
        + " }"
    else abort "generators.toPretty: should never happen (v = ${v})";

}