From e0a730695706696380b7f6413bb7b3048c7477ba Mon Sep 17 00:00:00 2001
From: c42f <chris42f@gmail.com>
Date: Fri, 17 Feb 2023 10:01:52 +1000
Subject: [PATCH 1/2] Prototype: Underscores and prefix anonymous function
 syntax

As discussed in https://github.com/JuliaLang/julia/issues/38713
---
 src/expr.jl   | 35 +++++++++++++++++++++++++++++++----
 src/parser.jl |  9 ++++++---
 2 files changed, 37 insertions(+), 7 deletions(-)

diff --git a/src/expr.jl b/src/expr.jl
index 61ec2e75..84503134 100644
--- a/src/expr.jl
+++ b/src/expr.jl
@@ -30,6 +30,27 @@ function reorder_parameters!(args, params_pos)
     insert!(args, params_pos, pop!(args))
 end
 
+function lower_underscores!(anon_args, args, argrange=1:length(args))
+    for i in argrange
+        a = args[i]
+        if a == :_
+            g = gensym()
+            push!(anon_args, g)
+            args[i] = g
+        elseif a isa Expr
+            if Meta.isexpr(a, :call) && length(a.args) > 2 &&
+                    Meta.isexpr(a.args[2], :parameters)
+                lower_underscores!(anon_args, a.args, 1:1)
+                lower_underscores!(anon_args, a.args, 3:length(a.args))
+                lower_underscores!(anon_args, a.args, 2:2)
+            else
+                # FIXME: Other out-of-source-order Exprs
+                lower_underscores!(anon_args, a.args)
+            end
+        end
+    end
+end
+
 function _to_expr(node::SyntaxNode; iteration_spec=false, need_linenodes=true,
                   eq_to_kw=false, map_kw_in_params=false)
     if !haschildren(node)
@@ -250,11 +271,17 @@ function _to_expr(node::SyntaxNode; iteration_spec=false, need_linenodes=true,
         # Block for conditional's source location
         args[1] = Expr(:block, loc, args[1])
     elseif headsym === :(->)
-        if Meta.isexpr(args[2], :block)
-            pushfirst!(args[2].args, loc)
+        if is_prefix_op_call(node)
+            anon_args = Symbol[]
+            lower_underscores!(anon_args, args)
+            pushfirst!(args, Expr(:tuple, anon_args...))
         else
-            # Add block for source locations
-            args[2] = Expr(:block, loc, args[2])
+            if Meta.isexpr(args[2], :block)
+                pushfirst!(args[2].args, loc)
+            else
+                # Add block for source locations
+                args[2] = Expr(:block, loc, args[2])
+            end
         end
     elseif headsym === :function
         if length(args) > 1
diff --git a/src/parser.jl b/src/parser.jl
index 6318054a..6d302455 100644
--- a/src/parser.jl
+++ b/src/parser.jl
@@ -276,7 +276,7 @@ function is_syntactic_operator(k)
 end
 
 function is_syntactic_unary_op(k)
-    kind(k) in KSet"$ & ::"
+    kind(k) in KSet"$ & :: ->"
 end
 
 function is_type_operator(t)
@@ -1417,6 +1417,7 @@ end
 # &a   ==>  (& a)
 # ::a  ==>  (::-pre a)
 # $a   ==>  ($ a)
+# ->a  ==>  (-> a)
 #
 # flisp: parse-unary-prefix
 function parse_unary_prefix(ps::ParseState)
@@ -1434,14 +1435,16 @@ function parse_unary_prefix(ps::ParseState)
             if k in KSet"& ::"
                 # &a   ==>  (& a)
                 parse_where(ps, parse_call)
+            elseif k == K"->"
+                # -> binds loosely on the right
+                parse_eq_star(ps)
             else
                 # $a   ==>  ($ a)
                 # $$a  ==>  ($ ($ a))
                 # $&a  ==>  ($ (& a))
                 parse_unary_prefix(ps)
             end
-            # Only need PREFIX_OP_FLAG for ::
-            f = k == K"::" ? PREFIX_OP_FLAG : EMPTY_FLAGS
+            f = (k == K"::" || k == K"->") ? PREFIX_OP_FLAG : EMPTY_FLAGS
             emit(ps, mark, k, f)
         end
     else

From c4e0ef59b77648fc3387da7d7fc1d3065b53769b Mon Sep 17 00:00:00 2001
From: c42f <chris42f@gmail.com>
Date: Sat, 18 Feb 2023 14:03:03 +1000
Subject: [PATCH 2/2] Make _ always refer to the same (single) argument

---
 src/expr.jl | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/src/expr.jl b/src/expr.jl
index 84503134..30cc3b9c 100644
--- a/src/expr.jl
+++ b/src/expr.jl
@@ -34,8 +34,12 @@ function lower_underscores!(anon_args, args, argrange=1:length(args))
     for i in argrange
         a = args[i]
         if a == :_
-            g = gensym()
-            push!(anon_args, g)
+            if isempty(anon_args)
+                g = gensym()
+                push!(anon_args, g)
+            else
+                g = anon_args[1]
+            end
             args[i] = g
         elseif a isa Expr
             if Meta.isexpr(a, :call) && length(a.args) > 2 &&