nix/tests/functional/lang/eval-okay-dynamic-attrs.nix

let
  aString = "a";

  bString = "b";
in {
  hasAttrs = { a.b = null; } ? "${aString}".b;

  selectAttrs = { a.b = true; }.a."${bString}";

  selectOrAttrs = { }."${aString}" or true;

  binds = { "${aString}"."${bString}c" = true; }.a.bc;

  recBinds = rec { "${bString}" = a; a = true; }.b;

  multiAttrs = { "${aString}" = true; "${bString}" = false; }.a;
}
Dynamic attrs This adds new syntax for attribute names: * attrs."${name}" => getAttr name attrs * attrs ? "${name}" => isAttrs attrs && hasAttr attrs name * attrs."${name}" or def => if attrs ? "${name}" then attrs."${name}" else def * { "${name}" = value; } => listToAttrs [{ inherit name value; }] Of course, it's a bit more complicated than that. The attribute chains can be arbitrarily long and contain combinations of static and dynamic parts (e.g. attrs."${foo}".bar."${baz}" or qux), which is relatively straightforward for the getAttrs/hasAttrs cases but is more complex for the listToAttrs case due to rules about duplicate attribute definitions. For attribute sets with dynamic attribute names, duplicate static attributes are detected at parse time while duplicate dynamic attributes are detected when the attribute set is forced. So, for example, { a = null; a.b = null; "${"c"}" = true; } will be a parse-time error, while { a = {}; "${"a"}".b = null; c = true; } will be an eval-time error (technically that case could theoretically be detected at parse time, but the general case would require full evaluation). Moreover, duplicate dynamic attributes are not allowed even in cases where they would be with static attributes ({ a.b.d = true; a.b.c = false; } is legal, but { a."${"b"}".d = true; a."${"b"}".c = false; } is not). This restriction might be relaxed in the future in cases where the static variant would not be an error, but it is not obvious that that is desirable. Finally, recursive attribute sets with dynamic attributes have the static attributes in scope but not the dynamic ones. So rec { a = true; "${"b"}" = a; } is equivalent to { a = true; b = true; } but rec { "${"a"}" = true; b = a; } would be an error or use a from the surrounding scope if it exists. Note that the getAttr, getAttr or default, and hasAttr are all implemented purely in the parser as syntactic sugar, while attribute sets with dynamic attribute names required changes to the AST to be implemented cleanly. This is an alternative solution to and closes #167 Signed-off-by: Shea Levy <shea@shealevy.com> 2013-09-21 03:25:30 +00:00			`let`
			`aString = "a";`

			`bString = "b";`
			`in {`
			`hasAttrs = { a.b = null; } ? "${aString}".b;`

			`selectAttrs = { a.b = true; }.a."${bString}";`

			`selectOrAttrs = { }."${aString}" or true;`

			`binds = { "${aString}"."${bString}c" = true; }.a.bc;`

			`recBinds = rec { "${bString}" = a; a = true; }.b;`

			`multiAttrs = { "${aString}" = true; "${bString}" = false; }.a;`
			`}`