Simplify indented output rendering by using stateful output routines.

This replaces the "pure functional" output style, which took great pains to
build a nested list structure which was only sent to the output device at the
very end.  This required passing around a hidden "tab" parameter in a monadic
fashion, the use of a "next" continuation parameter, and semicolons to ensure
that the tail of a structure was threaded in properly.

Now when you call routines like say, nl, put, and indent, the effects of those
calls are going directly into a file handle or other target as they run.  You
are no longer building up a lazy nested piece of data which is _later_ sent to
an output target.  All the complexity of the "pure" method vanishes like a bad
dream.

This drastically simplifies the code supporting the b15 and index_C tests.
I also use the technique to great advantage in the demo.ray project:

	https://github.com/chkoreff/demo.ray

This illustrates a principle which I title:

	"Pure Functions Sometimes Considered Harmful"

The pure functional style still has its place in certain aspects of a program,
but for input and output it's easier just to cut to the chase and deal with the
machine on its own terms.  In those cases, "var" is your friend -- and it can
even create more opportunities for implementing routines directly in C, as I
did with the parsing primitives in stream.c in the Fexl project.

I am also finding the var technique useful for "throughput".  In one major
project I was passing a single immutable state parameter around everywhere,
and I greatly simplified the code by keeping that state in a handful of vars.

The good thing about "var" is that it is _controlled_ mutability, not the
haphazard and dangerous kind where you modify nested data structures in place.
In Fexl the data structures are still immutable, but you use a var to _point_
to a new version when you make a state change.  I think this strikes the right
balance between pure functions and routines with side effects.

Author: chkoreff

src/VERSION

@@ -1 +1 @@
-27.11.0+1
+27.12.0

src/out/b15

@@ -208,7 +208,7 @@ get: GOOD
 {"bccc" "5233" "9999"}
 {"bcccxyz" "3364" void}
 ]
-steps 1645232 bytes 577120
+steps 1644462 bytes 577120
 ~
 \err=
 ~

src/test/lib/b15/context.fxl

@@ -15,29 +15,27 @@ cx_render_list;
 	is_defined val (fexl_quote val) "void"
 	)
 
-\render_pairs=
+\say_pairs=
 	(
-	render_list
-		(\row\next
+	say_list
+		(\row
 		row \key\val
-		say ["{" (fexl_quote key) " " (render_val val) "}"];
-		next
+		say ["{" (fexl_quote key) " " (render_val val) "}"]
 		)
 	)
 
-\render_errors=
+\say_errors=
 	(
-	render_list
-		(\row\next
+	say_list
+		(\row
 		row \key\val\val2
-		say ["{" (fexl_quote key)" " (render_val val)" "(render_val val2)"}"];
-		next
+		say ["{" (fexl_quote key)" " (render_val val)" "(render_val val2)"}"]
 		)
 	)
 
 \form
-def "render_pairs" render_pairs;
-def "render_errors" render_errors;
+def "say_pairs" say_pairs;
+def "say_errors" say_errors;
 form
 )
 
@@ -50,29 +48,10 @@ form
 	use "lib/index/render.fxl"
 	)
 
-\cx_output=
-(
-value;
-std;
-cx_render;
-cx_render_index;
+\cx_output==
+(\form
 cx_render_local;
-def "_put" put;
-\;
-
-\put_render=
-	(\render\tab\x
-	_put (render x end tab)
-	)
-
-\say_pairs=(put_render render_pairs "")
-\say_index=(put_render render_index "")
-\say_errors=(put_render render_errors "")
-
-\form
-def "say_pairs" say_pairs;
-def "say_index" say_index;
-def "say_errors" say_errors;
+cx_render_index;
 form
 )
 

src/test/lib/index/render.fxl

@@ -1,27 +1,21 @@
 #
-\render_index=
-	(@\render_index
-	render_list
-		(\row\next
+\say_index=
+	(@\say_index
+	say_list
+		(\row
 		row \is_leaf\key\val
 		is_leaf
 			(
-			say ["{T "(fexl_quote key) " " (fexl_quote val) "}"];
-			next
+			say ["{T "(fexl_quote key) " " (fexl_quote val) "}"]
 			)
 			(
-			say ["{F "(fexl_quote key) ];
-			indent
-				(
-				render_index val;
-				end
-				);
-			say "}";
-			next
+			say ["{F "(fexl_quote key)]
+			indent (say_index val)
+			say "}"
 			)
 		)
 	)
 
 \form
-def "render_index" render_index;
+def "say_index" say_index;
 form

src/test/lib/index_C/context.fxl

@@ -12,106 +12,79 @@ cx_render_C;
 
 # Process all the rows in an index.
 \do_rows=
-	(@\do_rows \checked_bound\pos\rows\next
-	rows next \row\rows
+	(@\do_rows \checked_bound\pos\rows
+	rows () \row\rows
 	row \is_leaf\key\val
 
 	\check_ch==
-		(\next
+		(
 		\ch=(slice key pos 1)
-		checked_bound () (say ["if (n <= "(num_str pos)") return 0;"]);
-		say ["if (x["(num_str pos)"] == '"ch"')"];
-		next
+		checked_bound () (say ["if (n <= "(num_str pos)") return 0;"])
+		say ["if (x["(num_str pos)"] == '"ch"')"]
 		)
 
 	\len=(length key)
 
 	is_leaf
 		(
 		\do_strncmp==
-			(\next
+			(
 			say ["if (n == "(num_str len)" && strncmp(x,"
-				(fexl_quote key)",n) == 0)"];
-			indent
-				(
-				say ["return "(fexl_quote val)";"];
-				end
-				);
-			next
+				(fexl_quote key)",n) == 0)"]
+			indent (say ["return "(fexl_quote val)";"])
 			)
 
 		lt pos len
 			(
-			check_ch;
+			check_ch
 			scope
-				(\next
-				do_strncmp;
-				say "return 0;";
-				next
-				);
-			do_rows T pos rows;
-			next
+				(
+				do_strncmp
+				say "return 0;"
+				)
+			do_rows T pos rows
 			);
 		eq len 0
 			(
 			# Optimize null key.
-			say ["if (n == "(num_str pos)")"];
-			indent
-				(
-				say ["return "(fexl_quote val)";"];
-				end
-				);
-			do_rows T pos rows;
-			next
+			say ["if (n == "(num_str pos)")"]
+			indent (say ["return "(fexl_quote val)";"])
+			do_rows T pos rows
 			)
 			(
-			do_strncmp;
-			do_rows checked_bound pos rows;
-			next
+			do_strncmp
+			do_rows checked_bound pos rows
 			)
 		)
 		(
-		check_ch;
+		check_ch
 		scope
-			(\next
-			do_rows F len val;
-			say "return 0;";
-			next
-			);
-		do_rows T pos rows;
-		next
+			(
+			do_rows F len val
+			say "return 0;"
+			)
+		do_rows T pos rows
 		)
 	)
 
-\render_index==
+# (compile_index index)
+# Return C code which implements the index.
+\compile_index==
 	(\index
-	say "const char *lookup(const char *x, unsigned long n)";
+	say "const char *lookup(const char *x, unsigned long n)"
 	scope
-		(\next
-		(\return
+		(
 		index
 			(
 			# Special case for empty index.
-			say "(void)x;";
-			say "(void)n;";
-			return
+			say "(void)x;"
+			say "(void)n;"
 			)
 			(\_\_
-			do_rows F 0 index;
-			return
+			do_rows F 0 index
 			)
-		);
-		say "return 0;";
-		next
-		);
-	end
-	)
-
-# (compile_index index)
-# Return C code which implements the index.
-\compile_index==
-	(\index
-	render_index index ""
+		say "return 0;"
+		)
 	)
 
 # (index_put_pairs pairs index)

src/test/lib/render/C.fxl

@@ -5,10 +5,9 @@
 	(\content
 	indent
 		(
-		say "{";
-		content;
-		say "}";
-		end
+		say "{"
+		content
+		say "}"
 		)
 	)
 

src/test/lib/render/base.fxl

@@ -1,19 +1,64 @@
-# Pure functional output
+# Indented output
 
-\end==(\tab [])
-\say==(\x\next\tab [tab x NL; next tab])
-\put==(\x\next\tab [x; next tab])
-\nl=(put NL)
+\v_put=var_new
+\v_nl=var_new
+\v_say=var_new
+\v_tab=var_new
 
+\put==(var_get v_put)
+\nl==(var_get v_nl ())
+\say==(var_get v_say)
+
+# Indent content with a new tab stop.
 \indent==
-	(\content\next\tab
-	[(eval (. tab TAB) content); next tab]
+	(\content
+	\tab=(var_get v_tab)
+	var_put v_tab (. tab TAB)
+	content
+	var_put v_tab tab
+	)
+
+# Slide over to the current tab stop.
+\tab==(put (var_get v_tab))
+
+# Redirect output functions to a file handle.
+\set_output_fh==
+	(\fh
+	var_put v_tab ""
+	var_put v_put (fput fh)
+	var_put v_nl (yield; fnl fh)
+	var_put v_say
+		(\x
+		fput fh (var_get v_tab)
+		fsay fh x
+		)
+	)
+
+# Run content with output functions sent to a file handle.
+\use_output_fh==
+	(\fh\content
+
+	\save_tab=(var_get v_tab)
+	\save_put=(var_get v_put)
+	\save_nl=(var_get v_nl)
+	\save_say=(var_get v_say)
+
+	set_output_fh fh
+	content
+
+	var_put v_tab save_tab
+	var_put v_put save_put
+	var_put v_nl save_nl
+	var_put v_say save_say
 	)
 
+set_output_fh stdout
+
 \form
-def "end" end;
-def "say" say;
 def "put" put;
 def "nl" nl;
+def "say" say;
 def "indent" indent;
+def "tab" tab;
+def "use_output_fh" use_output_fh;
 form

src/test/lib/render/list.fxl

@@ -1,21 +1,11 @@
 #
-\render_rows==
-	(\render_row @\loop \list\next
-	list next \row\tail
-	render_row row;
-	loop tail;
-	next
-	)
-
-\render_list==
-	(\render_row\list\next
-	say "[";
-	render_rows render_row list;
-	say "]";
-	next
+\say_list==
+	(\say_row\list
+	say "["
+	each list say_row
+	say "]"
 	)
 
 \form
-def "render_rows" render_rows;
-def "render_list" render_list;
+def "say_list" say_list;
 form