feat: add <<< and >>> abbreviating operations to default stream

Resolves #16.

syntax/fostr.sdf3

@@ -42,14 +42,18 @@ context-free syntax
   Ex.Stream    = <stream>
   Ex.Sum       = <<Ex> +  <Ex>> {left}
   Ex.Gets      = [[Ex] << [Ex]] {left}
+  Ex.DefGets   = [<<< [Ex]]
   Ex.To        = [[Ex] >> [Ex]] {left}
+  Ex.DefTo     = [[Ex] >>>]
 
   Ex           = <(<Ex>)> {bracket}
 
 context-free priorities
 
   Ex.To
+  > Ex.DefTo
   > Ex.Sum
+  > Ex.DefGets
   > Ex.Gets,
 
   // prevent cycle: no singletons

tests/basic.spt

@@ -231,3 +231,15 @@ run desugar-fostr to TopLevel(Sequence([
   Terminate(Sum(Int("11"), To(Int("12"), Stream()))),
   To(To(Int("13"), Stream()), Stream())
 ]))
+
+test emit_several_default [[
+<<< 1 + 2; 3 >>>
+(4 + 5) >>> >> stream; stream << 6;
+<<< 7 << 75
+<<< 8
+  + (9+10);
+11 + 12 >>>; 13 >>>
+                >>>
+]] parse succeeds
+/** writes
+3399677527121313*/

tests/emit_several.fos

@@ -1,7 +1,7 @@
-stream << 1 + 2; 3 >> stream
+<<< 1 + 2; 3 >>>
-(4 + 5) >> stream; stream << 6;
+(4 + 5) >>> >> stream; stream << 6;
-stream << 7 
+<<< 7 << 75
-stream << 8
+<<< 8
   + (9+10);
-11 + 12 >> stream; 13 >> stream
+11 + 12 >>>; 13 >>>
-                      >> stream
+                >>>

tests/hw.fos

@@ -1 +1 @@
-stream << 'Hello,	world!'
+<<< 'Hello, world!'

trans/desugar.str

@@ -15,7 +15,9 @@ rules
 
   seqFlatten: Sequence(l) -> Sequence(<mapconcat(?Sequence(<id>) <+ ![<id>])>l)
   
+  defStream:  DefGets(x)  -> Gets(Stream(), x)
+  defStream:  DefTo(x)    -> To(x, Stream())
+
 strategies
 
-  desugar-fostr = bottomup(try(deISe <+ seqFlatten))
+  desugar-fostr = bottomup(try(defStream <+ deISe <+ seqFlatten))
- 

trans/haskell.str

@@ -19,7 +19,8 @@ rules
      returning the final value.
    */
 
-  hs: (_, TopLevel(val)) -> $[import System.IO
+  hs: (_, TopLevel(val)) -> $[-- Preamble from fostr
+  import System.IO
   data IOStream = StdIO
 
   gets :: Show b => a -> b -> IO a

trans/javascript.str

@@ -2,13 +2,16 @@ module javascript
 imports libstrategolib signatures/- util
 
 rules
-  js: TopLevel(x) -> $[const Stdio = {
+  js: TopLevel(x) -> $[// Fostr preamble
+  const Stdio = {
     gets: v => { process.stdout.write(String(v)); return Stdio; },
   }
   function to(data, strm) {
     strm.gets(data);
     return data;
   }
+  // End of preamble
+
   [x]]
 
   js: Stream()     -> $[Stdio]

trans/python.str

@@ -2,7 +2,8 @@ module python
 imports libstrategolib signatures/- util
 rules
 
-  py: TopLevel(x) -> $[import sys
+  py: TopLevel(x) -> $[## Fostr preamble
+  import sys
   class StdioC:
     def gets(self, v):
       print(v, file=sys.stdout, end='')
@@ -11,6 +12,8 @@ rules
     strm.gets(data)
     return data
   Stdio = StdioC()
+  ## End of preamble
+
   [x]]
 
   py: Stream()     -> $[Stdio]