Change how -> is represented in the AST

The previous representation was very hard to compose
and very often required the AST nodes to be created
manually.

The current approach is easier to compose because
the node is represented as any other operator as long
as it is wrapped in parentheses.

For example, imagine we have pairs of patterns and
the code to be executed for that pattern, and we want
to inject them into a case. It can now be writen as:

    pairs = Enum.map pairs, fn { pattern, expr } ->
      quote do: (unquote(pattern) -> unquote(expr))
    end

    quote do
      case unquote(condition), do: unquote(pairs)
    end

Compare to the previous implementation:

    pairs = Enum.map pairs, fn { pattern, expr } ->
      { [pattern], [], expr }
    end

    pairs = { :->, [], pairs }

    quote do
      case unquote(condition), do: unquote(pairs)
    end

Author: José Valim

lib/elixir/lib/kernel.ex

@@ -2173,8 +2173,8 @@ defmodule Kernel do
           "This will"
       end
   """
-  defmacro cond([do: { :->, _, pairs }]) do
-    [{ [condition], meta, clause }|t] = :lists.reverse pairs
+  defmacro cond([do: pairs]) do
+    [{ :->, meta, [[condition], clause] }|t] = :lists.reverse pairs
 
     new_acc =
       case condition do
@@ -2205,22 +2205,20 @@ defmodule Kernel do
   #         end
   #     end
   #
-  defp build_cond_clauses([{ [condition], new, clause }|t], acc, old) do
-    stab = { :->, [], [falsy_clause(old, acc), truthy_clause(new, clause)] }
-    acc  = quote do
-      case unquote(condition), do: unquote(stab)
-    end
+  defp build_cond_clauses([{ :->, new, [[condition], clause] }|t], acc, old) do
+    clauses = [falsy_clause(old, acc), truthy_clause(new, clause)]
+    acc = quote do: (case unquote(condition), do: unquote(clauses))
     build_cond_clauses(t, acc, new)
   end
 
   defp build_cond_clauses([], acc, _), do: acc
 
   defp falsy_clause(meta, acc) do
-    { [quote(do: unquote(cond_var) when unquote(cond_var) in [false, nil])], meta, acc }
+    { :->, meta, [[quote(do: unquote(cond_var) when unquote(cond_var) in [false, nil])], acc] }
   end
 
   defp truthy_clause(meta, clause) do
-    { [quote(do: _)], meta, clause }
+    { :->, meta, [[quote(do: _)], clause] }
   end
 
   # Setting cond: true in metadata turns on a small optimization

lib/elixir/lib/kernel/typespec.ex

@@ -552,11 +552,11 @@ defmodule Kernel.Typespec do
 
   defp typespec_to_ast({ :type, line, :fun, [{:type, _, :product, args}, result] }) do
     args = lc arg inlist args, do: typespec_to_ast(arg)
-    { :->, [line: line], [{args, [line: line], typespec_to_ast(result)}] }
+    [{ :->, [line: line], [args, typespec_to_ast(result)] }]
   end
 
   defp typespec_to_ast({ :type, line, :fun, [args, result] }) do
-    { :->, [line: line], [{[typespec_to_ast(args)], [line: line], typespec_to_ast(result)}] }
+    [{ :->, [line: line], [[typespec_to_ast(args)], typespec_to_ast(result)] }]
   end
 
   defp typespec_to_ast({ :type, line, :fun, [] }) do
@@ -669,11 +669,7 @@ defmodule Kernel.Typespec do
   end
 
   # Handle funs
-  defp typespec({:->, meta, [{[{:fun, _, arguments}], cmeta, return}]}, vars, caller) when is_list(arguments) do
-    typespec({:->, meta, [{arguments, cmeta, return}]}, vars, caller)
-  end
-
-  defp typespec({:->, meta, [{arguments, _, return}]}, vars, caller) when is_list(arguments) do
+  defp typespec([{:->, meta, [arguments, return]}], vars, caller) when is_list(arguments) do
     args = fn_args(meta, arguments, return, vars, caller)
     { :type, line(meta), :fun, args }
   end

lib/elixir/lib/macro.ex

@@ -5,8 +5,11 @@ defmodule Macro do
   Conveniences for working with macros.
   """
 
-  @typedoc "Abstract Syntax Tree (AST) node"
-  @type t :: { t, t } | { t, Keyword.t, t } | atom | number | binary | list
+  @typedoc "Abstract Syntax Tree (AST)"
+  @type t :: node | { t, t } | atom | number | binary | [t]
+
+  @typedoc "The AST node (remaining ones are literals)"
+  @type node :: { node | atom, Keyword.t, atom | [t] }
 
   @binary_ops [ :===, :!==,
     :==, :!=, :<=, :>=,
@@ -309,22 +312,22 @@ defmodule Macro do
   end
 
   # Fn keyword
-  def to_string({ :fn, _, [{ :->, _, [{_, _, tuple}] } = arrow] } = ast, fun)
+  def to_string({ :fn, _, [{ :->, _, [_, tuple] }] = arrow } = ast, fun)
       when not is_tuple(tuple) or elem(tuple, 0) != :__block__ do
     fun.(ast, "fn " <> arrow_to_string(arrow, fun) <> " end")
   end
 
-  def to_string({ :fn, _, [{ :->, _, [_] } = block] } = ast, fun) do
+  def to_string({ :fn, _, [{ :->, _, _ }] = block } = ast, fun) do
     fun.(ast, "fn " <> block_to_string(block, fun) <> "\nend")
   end
 
-  def to_string({ :fn, _, [block] } = ast, fun) do
+  def to_string({ :fn, _, block } = ast, fun) do
     block = adjust_new_lines block_to_string(block, fun), "\n  "
     fun.(ast, "fn\n  " <> block <> "\nend")
   end
 
   # left -> right
-  def to_string({ :->, _, _ } = ast, fun) do
+  def to_string([{ :->, _, _ }|_] = ast, fun) do
     fun.(ast, "(" <> arrow_to_string(ast, fun, true) <> ")")
   end
 
@@ -442,8 +445,8 @@ defmodule Macro do
     atom_to_binary(key) <> "\n  " <> block <> "\n"
   end
 
-  defp block_to_string({ :->, _, exprs }, fun) do
-    Enum.map_join(exprs, "\n", fn({ left, _, right }) ->
+  defp block_to_string([{ :->, _, _ }|_] = block, fun) do
+    Enum.map_join(block, "\n", fn({ :->, _, [left, right] }) ->
       left = comma_join_or_empty_paren(left, fun, false)
       left <> "->\n  " <> adjust_new_lines block_to_string(right, fun), "\n  "
     end)
@@ -483,8 +486,8 @@ defmodule Macro do
 
   defp op_to_string(expr, fun, _, _), do: to_string(expr, fun)
 
-  defp arrow_to_string({ :->, _, pairs }, fun, paren // false) do
-    Enum.map_join(pairs, "; ", fn({ left, _, right }) ->
+  defp arrow_to_string(pairs, fun, paren // false) do
+    Enum.map_join(pairs, "; ", fn({ :->, _, [left, right] }) ->
       left = comma_join_or_empty_paren(left, fun, paren)
       left <> "-> " <> to_string(right, fun)
     end)
@@ -707,7 +710,9 @@ defmodule Macro do
     { tree, cache }
   end
 
-  @doc false # Used internally by Elixir
+  # TODO: Rewrite this function to use elixir_exp
+  # For such, there is no need for a cache.
+  @doc false
   def expand_all(tree, env, cache) do
     expand_all_until(expand(tree, env, cache), env)
   end

lib/elixir/src/elixir_clauses.erl

@@ -13,16 +13,24 @@ get_pairs(Meta, Key, Clauses, S) ->
 
 get_pairs(Meta, Key, Clauses, S, AllowNil) ->
   case lists:keyfind(Key, 1, Clauses) of
-    { Key, { '->', _, Pairs } } ->
-      [{ Key, PMeta, Left, Right } || { Left, PMeta, Right } <- Pairs];
+    { Key, Pairs } when is_list(Pairs) ->
+      [get_pair(Meta, Key, Pair, S) || Pair <- Pairs];
     { Key, nil } when AllowNil ->
       [];
     { Key, _ } ->
-      elixir_errors:syntax_error(Meta, S#elixir_scope.file, "expected pairs with -> for key ~ts", [Key]);
+      get_pairs_error(Meta, Key, S);
     _ ->
       []
   end.
 
+get_pair(_Meta, Key, { '->', Meta, [Left, Right] }, _S) ->
+  { Key, Meta, Left, Right };
+get_pair(Meta, Key, _Other, S) ->
+  get_pairs_error(Meta, Key, S).
+
+get_pairs_error(Meta, Key, S) ->
+  elixir_errors:compile_error(Meta, S#elixir_scope.file, "expected -> clauses for key ~ts", [Key]).
+
 % Function for translating assigns.
 
 assigns(Fun, Args, #elixir_scope{context=Context, temp_vars=TempVars,

lib/elixir/src/elixir_exp.erl

@@ -203,6 +203,10 @@ expand({ quote, Meta, [_, _] }, E) ->
 
 %% Functions
 
+%% TODO: Remove me. Temporary during refactoring.
+expand({ '&', _, [Arg] } = Original, E) when is_integer(Arg) ->
+  { Original, E };
+
 expand({ '&', Meta, [Arg] }, E) ->
   % assert_no_match_or_guard_scope(Meta, '&', S),
   case elixir_fn:capture(Meta, Arg, E) of

lib/elixir/src/elixir_fn.erl

@@ -5,7 +5,7 @@
 -include("elixir.hrl").
 
 fn(Meta, Clauses, S) ->
-  Transformer = fun({ ArgsWithGuards, CMeta, Expr }, Acc) ->
+  Transformer = fun({ '->', CMeta, [ArgsWithGuards, Expr] }, Acc) ->
     { Args, Guards } = elixir_clauses:extract_splat_guards(ArgsWithGuards),
     elixir_clauses:assigns_block(?line(CMeta), fun translate_fn_match/2, Args, [Expr], Guards, umergec(S, Acc))
   end,
@@ -25,6 +25,8 @@ translate_fn_match(Arg, S) ->
   { TArg, TS } = elixir_translator:translate(Arg, S#elixir_scope{extra=fn_match}),
   { TArg, TS#elixir_scope{extra=S#elixir_scope.extra} }.
 
+%% Capture
+
 capture(Meta, { '/', _, [{ { '.', _, [_, F] } = Dot, RequireMeta , [] }, A] }, E) when is_atom(F), is_integer(A) ->
   Args = [{ '&', [], [X] } || X <- lists:seq(1, A)],
   capture_require(Meta, { Dot, RequireMeta, Args }, E, true);
@@ -97,7 +99,7 @@ do_capture(Meta, Expr, E, Sequential) ->
       invalid_capture(Meta, Expr, E);
     { EExpr, EDict } ->
       EVars = validate(Meta, EDict, 1, E),
-      Fn = { fn, Meta, [{ '->', [], [{ EVars, Meta, EExpr }]}]},
+      Fn = { fn, Meta, [{ '->', Meta, [EVars, EExpr]}]},
       { expanded, Fn, E#elixir_env{macro_counter=E#elixir_env.macro_counter+1} }
   end.
 

lib/elixir/src/elixir_parser.yrl

@@ -578,7 +578,7 @@ extract_identifier(Other) -> Other.
 %% Fn
 
 build_fn(Op, Stab) ->
-  { fn, meta(Op), [Stab] }.
+  { fn, meta(Op), Stab }.
 
 %% Access
 
@@ -632,20 +632,20 @@ string_tokens_parse(Line, Tokens) ->
 %% Keywords
 
 build_stab([{ '->', Meta, [Left, Right] }|T]) ->
-  { '->', Meta, build_stab(Meta, T, Left, [Right], []) };
+  build_stab(Meta, T, Left, [Right], []);
 
 build_stab(Else) ->
   build_block(Else).
 
 build_stab(Old, [{ '->', New, [Left, Right] }|T], Marker, Temp, Acc) ->
-  H = { Marker, Old, build_block(lists:reverse(Temp)) },
+  H = { '->', Old, [Marker, build_block(lists:reverse(Temp))] },
   build_stab(New, T, Left, [Right], [H|Acc]);
 
 build_stab(Meta, [H|T], Marker, Temp, Acc) ->
   build_stab(Meta, T, Marker, [H|Temp], Acc);
 
 build_stab(Meta, [], Marker, Temp, Acc) ->
-  H = { Marker, Meta, build_block(lists:reverse(Temp)) },
+  H = { '->', Meta, [Marker, build_block(lists:reverse(Temp))] },
   lists:reverse([H|Acc]).
 
 %% Every time the parser sees a (unquote_splicing())

lib/elixir/src/elixir_translator.erl

@@ -243,9 +243,9 @@ translate_each({ '&', Meta, [Arg] }, S) ->
       translate_each(TE, S#elixir_scope{macro_counter=MacroCounter})
   end;
 
-translate_each({ fn, Meta, [{ '->', _, Pairs }] }, S) ->
+translate_each({ fn, Meta, Clauses }, S) ->
   assert_no_match_or_guard_scope(Meta, 'fn', S),
-  elixir_fn:fn(Meta, Pairs, S);
+  elixir_fn:fn(Meta, Clauses, S);
 
 %% Case
 

lib/elixir/test/elixir/kernel/quote_test.exs

@@ -112,22 +112,22 @@ defmodule Kernel.QuoteTest do
   end
 
   test :when do
-    assert {:->,_,[{[{:when,_,[1,2,3,4]}],_,5}]} = quote(do: (1, 2, 3 when 4 -> 5))
-    assert {:->,_,[{[{:when,_,[1,2,3,4]}],_,5}]} = quote(do: ((1, 2, 3) when 4 -> 5))
+    assert [{:->,_,[[{:when,_,[1,2,3,4]}],5]}] = quote(do: (1, 2, 3 when 4 -> 5))
+    assert [{:->,_,[[{:when,_,[1,2,3,4]}],5]}] = quote(do: ((1, 2, 3) when 4 -> 5))
 
-    assert {:->,_,[{[{:when,_,[1,2,3,{:when,_,[4,5]}]}],_,6}]} =
+    assert [{:->,_,[[{:when,_,[1,2,3,{:when,_,[4,5]}]}],6]}] =
              quote(do: ((1, 2, 3) when 4 when 5 -> 6))
   end
 
   test :stab do
-    assert { :->, _, [{[], _, _}] } = (quote do -> end)
-    assert { :->, _, [{[], _, _}] } = (quote do: (->))
+    assert [{ :->, _, [[], nil] }] = (quote do -> end)
+    assert [{ :->, _, [[], nil] }] = (quote do: (->))
 
-    assert { :->, _, [{[1], _, _}] } = (quote do 1 -> end)
-    assert { :->, _, [{[1], _, _}] } = (quote do: (1 ->))
+    assert [{ :->, _, [[1], nil] }] = (quote do 1 -> end)
+    assert [{ :->, _, [[1], nil] }] = (quote do: (1 ->))
 
-    assert { :->, _, [{[], _, 1}] } = (quote do -> 1 end)
-    assert { :->, _, [{[], _, 1}] } = (quote do: (-> 1))
+    assert [{ :->, _, [[], 1] }] = (quote do -> 1 end)
+    assert [{ :->, _, [[], 1] }] = (quote do: (-> 1))
   end
 
   test :bind_quoted do