feat WIP: checkpoint initial efforts on stream extraction

Adds ! operator and !! expression, with Python code gen and
  an item in tour.
  Still needs: other languages, check generated docs, and addition
  of standard input to test scheme.

syntax/fostr.sdf3

@@ -42,10 +42,13 @@ context-free syntax
   Ex.EscString = STRING
   Ex.Stream    = <stream>
   Ex.Sum       = <<Ex> +  <Ex>> {left}
+  Ex.Concat    = <<Ex> ++ <Ex>> {left}
   Ex.Gets      = [[Ex] << [Ex]] {left}
   Ex.DefGets   = [<<< [Ex]]
   Ex.To        = [[Ex] >> [Ex]] {left}
   Ex.DefTo     = [[Ex] >>>]
+  Ex.Emits     = <<Ex>!>
+  Ex.DefEmits  = <!!>
 
   Ex           = <(<Ex>)> {bracket}
 
@@ -53,7 +56,7 @@ context-free priorities
 
   Ex.To
   > Ex.DefTo
-  > Ex.Sum
+  > {Ex.Sum Ex.Concat}
   > Ex.DefGets
   > Ex.Gets,
 

tests/basic.spt

@@ -294,3 +294,36 @@ test emit_several_default [[
 ]] parse succeeds
 /** writes
 3399677527121313*/
+
+/** md
+### Streams are bidirectional
+
+So far we have only sent items to a stream. But we can extract them from
+streams as well, with the `!` postfix operator. `!!` all by itself abbreviates
+`stream!`, i.e., extraction from the standard stream. For example,
+
+```fostr
+**/
+
+/** md */ test custom_hw [[
+<<< "What is your name?\n"
+<<< 'Hello, ' ++ !!
+]] /* **/
+parse to TopLevel(Sequence([
+  DefGets(EscString("\"What is your name?\n\"")),
+  DefGets(Concat(LitString("'Hello, '"),DefEmits()))
+]))
+/** accepts
+Kilroy
+**/
+/** writes
+What is your name?
+Hello, Kilroy**/
+
+/** md
+```
+
+queries users for their name and then writes a customized greeting. It also
+illustratesthe use of `++` for string concatenation, as opposed to `+` for
+(numerical) addition.
+**/

trans/desugar.str

@@ -17,6 +17,7 @@ rules
   
   defStream:  DefGets(x)  -> Gets(Stream(), x)
   defStream:  DefTo(x)    -> To(x, Stream())
+  defStream:  DefEmits()  -> Emits(Stream())
 
 strategies
 

trans/python.str

@@ -8,6 +8,8 @@ rules
     def gets(self, v):
       print(v, file=sys.stdout, end='')
       return self
+    def emit(self):
+      return input()
   def to(data,strm):
     strm.gets(data)
     return data
@@ -21,8 +23,10 @@ rules
   py: LitString(x) -> $[r[x]]
   py: EscString(x) -> x
   py: Sum(x,y)     -> $[[x] + [y]]
+  py: Concat(x,y)  -> $[[x] + [y]]
   py: Gets(x, y)   -> $[[x].gets([y])]
   py: To(x, y)     -> $[to([x],[y])]
+  py: Emits(x)     -> $[[x].emit()]
   py: Terminate(x) -> $[[x];]
   py: Sequence(l)  -> <join(|"\n")>l
 

trans/statics.stx

@@ -222,6 +222,13 @@ This pattern lets us specify error messages.
     type_Ex(e2) == STREAM() | error $[Items may only be sent to Streams.]@e2.
   /* **/
 
+  ty_Ex(Concat(e1, e2)) = STRING() :-
+    type_Ex(e1) == STRING() | error $[Expression [e1] not String in concat.]@e1,
+    type_Ex(e2) == STRING() | error $[Expression [e2] not String in concat.]@e2.
+
+  ty_Ex(Emits(e)) = STRING() :- // At the moment, only stream is stdio
+    type_Ex(e) == STREAM() | error $[Only Streams may emit items.]@e.
+
   /** md
 ```