EEP 37: Funs with names

This adds optional names to fun expressions. A named fun expression
is parsed as a tuple `{named_fun,Loc,Name,Clauses}` in erl_parse.

If a fun expression has a name, it must be present and be the same in
every of its clauses. The function name shadows the environment of the
expression shadowing the environment and it is shadowed by the
environment of the clauses' arguments. An unused function name triggers
a warning unless it is prefixed by _, just as every variable.
Variable _ is allowed as a function name.

It is not an error to put a named function in a record field default
value.

When transforming to Core Erlang, the named fun Fun is changed into
the following expression:

    letrec 'Fun'/Arity =
               fun (Args) ->
                       let <Fun> = 'Fun'/Arity
                       in Case
    in 'Fun'/Arity

where Args is the list of arguments of 'Fun'/Arity and Case the
Core Erlang expression corresponding to the clauses of Fun.

This transformation allows us to entirely skip any k_var to k_local
transformation in the fun's clauses bodies.

8 files changed, 67 insertions, 5 deletions

diff --git a/lib/stdlib/examples/erl_id_trans.erl b/lib/stdlib/examples/erl_id_trans.erl
index 51def8c8e1..2c842fafc7 100644
--- a/lib/stdlib/examples/erl_id_trans.erl
+++ b/lib/stdlib/examples/erl_id_trans.erl
@@ -419,6 +419,8 @@ expr({'fun',Line,Body}) ->
 	    A = expr(A0),
 	    {'fun',Line,{function,M,F,A}}
     end;
+expr({named_fun,Loc,Name,Cs}) ->
+    {named_fun,Loc,Name,fun_clauses(Cs)};
 expr({call,Line,F0,As0}) ->
     %% N.B. If F an atom then call to local function or BIF, if F a
     %% remote structure (see below) then call to other module,
diff --git a/lib/stdlib/src/epp.erl b/lib/stdlib/src/epp.erl
index dd0512be4d..4fd302e612 100644
--- a/lib/stdlib/src/epp.erl
+++ b/lib/stdlib/src/epp.erl
@@ -1247,6 +1247,8 @@ macro_arg([{'case',Lc}|Toks], E, Arg) ->
     macro_arg(Toks, ['end'|E], [{'case',Lc}|Arg]);
 macro_arg([{'fun',Lc}|[{'(',_}|_]=Toks], E, Arg) ->
     macro_arg(Toks, ['end'|E], [{'fun',Lc}|Arg]);
+macro_arg([{'fun',_}=Fun,{var,_,_}=Name|[{'(',_}|_]=Toks], E, Arg) ->
+    macro_arg(Toks, ['end'|E], [Name,Fun|Arg]);
 macro_arg([{'receive',Lr}|Toks], E, Arg) ->
     macro_arg(Toks, ['end'|E], [{'receive',Lr}|Arg]);
 macro_arg([{'try',Lr}|Toks], E, Arg) ->
diff --git a/lib/stdlib/src/erl_expand_records.erl b/lib/stdlib/src/erl_expand_records.erl
index d05f630d8e..776b433613 100644
--- a/lib/stdlib/src/erl_expand_records.erl
+++ b/lib/stdlib/src/erl_expand_records.erl
@@ -344,6 +344,9 @@ expr({'fun',_,{function,_M,_F,_A}}=Fun, St) ->
 expr({'fun',Line,{clauses,Cs0}}, St0) ->
     {Cs,St1} = clauses(Cs0, St0),
     {{'fun',Line,{clauses,Cs}},St1};
+expr({named_fun,Line,Name,Cs0}, St0) ->
+    {Cs,St1} = clauses(Cs0, St0),
+    {{named_fun,Line,Name,Cs},St1};
 expr({call,Line,{atom,_,is_record},[A,{atom,_,Name}]}, St) ->
     record_test(Line, A, Name, St);
 expr({call,Line,{remote,_,{atom,_,erlang},{atom,_,is_record}},
diff --git a/lib/stdlib/src/erl_lint.erl b/lib/stdlib/src/erl_lint.erl
index bcf3ccef3b..cf01e1f8cf 100644
--- a/lib/stdlib/src/erl_lint.erl
+++ b/lib/stdlib/src/erl_lint.erl
@@ -2030,6 +2030,15 @@ expr({'fun',Line,Body}, Vt, St) ->
 	    {Bvt, St1} = expr_list([M,F,A], Vt, St),
 	    {vtupdate(Bvt, Vt),St1}
     end;
+expr({named_fun,_,'_',Cs}, Vt, St) ->
+    fun_clauses(Cs, Vt, St);
+expr({named_fun,Line,Name,Cs}, Vt, St0) ->
+    Nvt0 = [{Name,{bound,unused,[Line]}}],
+    St1 = shadow_vars(Nvt0, Vt, 'named fun', St0),
+    Nvt1 = vtupdate(vtsubtract(Vt, Nvt0), Nvt0),
+    {Csvt,St2} = fun_clauses(Cs, Nvt1, St1),
+    {_,St3} = check_unused_vars(vtupdate(Csvt, Nvt0), [], St2),
+    {vtold(Csvt, Vt),St3};
 expr({call,_Line,{atom,_Lr,is_record},[E,{atom,Ln,Name}]}, Vt, St0) ->
     {Rvt,St1} = expr(E, Vt, St0),
     {Rvt,exist_record(Ln, Name, St1)};
@@ -2182,6 +2191,7 @@ is_valid_record(Rec) ->
         {lc, _, _, _} -> false;
         {record_index, _, _, _} -> false;
         {'fun', _, _} -> false;
+        {named_fun, _, _, _} -> false;
         _ -> true
     end.
 
diff --git a/lib/stdlib/src/erl_parse.yrl b/lib/stdlib/src/erl_parse.yrl
index 7145b0858f..59a05a48ee 100644
--- a/lib/stdlib/src/erl_parse.yrl
+++ b/lib/stdlib/src/erl_parse.yrl
@@ -406,6 +406,9 @@ fun_clause -> argument_list clause_guard clause_body :
 	{Args,Pos} = '$1',
 	{clause,Pos,'fun',Args,'$2','$3'}.
 
+fun_clause -> var argument_list clause_guard clause_body :
+	{clause,element(2, '$1'),element(3, '$1'),element(1, '$2'),'$3','$4'}.
+
 try_expr -> 'try' exprs 'of' cr_clauses try_catch :
 	build_try(?line('$1'),'$2','$4','$5').
 try_expr -> 'try' exprs try_catch :
@@ -799,8 +802,15 @@ build_rule(Cs) ->
 %% build_fun(Line, [Clause]) -> {'fun',Line,{clauses,[Clause]}}.
 
 build_fun(Line, Cs) ->
+    Name = element(3, hd(Cs)),
     Arity = length(element(4, hd(Cs))),
-    {'fun',Line,{clauses,check_clauses(Cs, 'fun', Arity)}}.
+    CheckedCs = check_clauses(Cs, Name, Arity),
+    case Name of
+        'fun' ->
+            {'fun',Line,{clauses,CheckedCs}};
+        Name ->
+            {named_fun,Line,Name,CheckedCs}
+    end.
 
 check_clauses(Cs, Name, Arity) ->
      mapl(fun ({clause,L,N,As,G,B}) when N =:= Name, length(As) =:= Arity ->
diff --git a/lib/stdlib/src/erl_pp.erl b/lib/stdlib/src/erl_pp.erl
index 657cb5d34c..8a1d8e0440 100644
--- a/lib/stdlib/src/erl_pp.erl
+++ b/lib/stdlib/src/erl_pp.erl
@@ -511,10 +511,17 @@ lexpr({'fun',_,{function,M,F,A}}, _Prec, Opts) ->
     ArityItem = lexpr(A, Opts),
     ["fun ",NameItem,$:,CallItem,$/,ArityItem];
 lexpr({'fun',_,{clauses,Cs}}, _Prec, Opts) ->
-    {list,[{first,'fun',fun_clauses(Cs, Opts)},'end']};
+    {list,[{first,'fun',fun_clauses(Cs, Opts, unnamed)},'end']};
+lexpr({named_fun,_,Name,Cs}, _Prec, Opts) ->
+    {list,[{first,['fun', " "],fun_clauses(Cs, Opts, {named, Name})},'end']};
 lexpr({'fun',_,{clauses,Cs},Extra}, _Prec, Opts) ->
     {force_nl,fun_info(Extra),
-     {list,[{first,'fun',fun_clauses(Cs, Opts)},'end']}};
+     {list,[{first,'fun',fun_clauses(Cs, Opts, unnamed)},'end']}};
+lexpr({named_fun,_,Name,Cs,Extra}, _Prec, Opts) ->
+    {force_nl,fun_info(Extra),
+     {list,[{first,['fun', " "],fun_clauses(Cs, Opts, {named, Name})},'end']}};
+lexpr({'query',_,Lc}, _Prec, Opts) ->
+    {list,[{step,leaf("query"),lexpr(Lc, 0, Opts)},'end']};
 lexpr({call,_,{remote,_,{atom,_,M},{atom,_,F}=N}=Name,Args}, Prec, Opts) ->
     case erl_internal:bif(M, F, length(Args)) of
         true ->
@@ -729,8 +736,13 @@ stack_backtrace(S, El, Opts) ->
 %% fun_clauses(Clauses, Opts) -> [Char].
 %%  Print 'fun' clauses.
 
-fun_clauses(Cs, Opts) ->
-    nl_clauses(fun fun_clause/2, [$;], Opts, Cs).
+fun_clauses(Cs, Opts, unnamed) ->
+    nl_clauses(fun fun_clause/2, [$;], Opts, Cs);
+fun_clauses(Cs, Opts, {named, Name}) ->
+    nl_clauses(fun (C, H) ->
+                       {step,Gl,Bl} = fun_clause(C, H),
+                       {step,[atom_to_list(Name),Gl],Bl}
+               end, [$;], Opts, Cs).
 
 fun_clause({clause,_,A,G,B}, Opts) ->
     El = args(A, Opts),
diff --git a/lib/stdlib/src/ms_transform.erl b/lib/stdlib/src/ms_transform.erl
index 4e2ce39ec2..25b04fe45e 100644
--- a/lib/stdlib/src/ms_transform.erl
+++ b/lib/stdlib/src/ms_transform.erl
@@ -369,6 +369,13 @@ copy({var,_Line,Name} = VarDef,Bound) ->
 copy({'fun',Line,{clauses,Clauses}},Bound) -> % Dont export bindings from funs
     {NewClauses,_IgnoredBindings} = copy_list(Clauses,Bound),
     {{'fun',Line,{clauses,NewClauses}},Bound};
+copy({named_fun,Line,Name,Clauses},Bound) -> % Dont export bindings from funs
+    Bound1 = case Name of
+                 '_' -> Bound;
+                 Name -> gb_sets:add(Name,Bound)
+             end,
+    {NewClauses,_IgnoredBindings} = copy_list(Clauses,Bound1),
+    {{named_fun,Line,Name,NewClauses},Bound};
 copy({'case',Line,Of,ClausesList},Bound) -> % Dont export bindings from funs
     {NewOf,NewBind0} = copy(Of,Bound),
     {NewClausesList,NewBindings} = copy_case_clauses(ClausesList,NewBind0,[]),
diff --git a/lib/stdlib/src/qlc_pt.erl b/lib/stdlib/src/qlc_pt.erl
index 26bc4d1bdf..c26764eb18 100644
--- a/lib/stdlib/src/qlc_pt.erl
+++ b/lib/stdlib/src/qlc_pt.erl
@@ -2540,6 +2540,19 @@ nos({'fun',L,{clauses,Cs}}, S) ->
                {clause,Ln,H,G,B}
            end || {clause,Ln,H0,G0,B0} <- Cs],
     {{'fun',L,{clauses,NCs}}, S};
+nos({named_fun,Loc,Name,Cs}, S) ->
+    {{var,NLoc,NName}, S1} = case Name of
+                                 '_' ->
+                                     S;
+                                 Name ->
+                                     nos_pattern({var,Loc,Name}, S)
+                               end,
+    NCs = [begin
+               {H, S2} = nos_pattern(H0, S1),
+               {[G, B], _} = nos([G0, B0], S2),
+               {clause,CLoc,H,G,B}
+           end || {clause,CLoc,H0,G0,B0} <- Cs],
+    {{named_fun,NLoc,NName,NCs}, S};
 nos({lc,L,E0,Qs0}, S) ->
     %% QLCs as well as LCs. It is OK to modify LCs as long as they
     %% occur within QLCs--the warning messages have already been found
@@ -2713,6 +2726,9 @@ var2const(E) ->
 
 var_map(F, {var, _, _}=V) ->
     F(V);
+var_map(F, {named_fun,NLoc,NName,Cs}) ->
+    {var,Loc,Name} = F({var,NLoc,NName}),
+    {named_fun,Loc,Name,var_map(F, Cs)};
 var_map(F, T) when is_tuple(T) ->
     list_to_tuple(var_map(F, tuple_to_list(T)));
 var_map(F, [E | Es]) ->