Merge pull request 'docs: Finally get the tour to start from the real helloworld' (#23) from hw_tour into main

Also improves the testing situation for the features to date.
  Resolves #17.

Reviewed-on: #23

tests/basic.spt

@@ -16,15 +16,21 @@ Title: A whirlwind tour of fostr
 
 ## Whirlwind tour
 
-fostr is just in its infancy, so it's not yet even ready for
-Hello, World. The best we can offer now is this little snippet
-that writes the sum of the ASCII codes for 'H', 'W', and '!' to standard output:
+There seems only to be one way to start a tour like this. So here goes:
 ```fostr
 **/
 
-/** md */ test emit_sum [[
-stream << 72 + 87 + 33
+/** md */ test hello_world [[
+<<< 'Hello, world!'
 ]] /* **/
+parse to TopLevel(DefGets(LitString("'Hello, world!'")))
+/** writes
+Hello, world!**/
+
+// Prior proto-hello-world, no longer in the tour.
+test emit_sum [[
+stream << 72 + 87 + 33
+]]
 parse to TopLevel(Gets(Stream(), Sum(Sum(Int("72"), Int("87")), Int("33"))))
 /** writes
 192**/
@@ -33,7 +39,7 @@ parse to TopLevel(Gets(Stream(), Sum(Sum(Int("72"), Int("87")), Int("33"))))
 ```
 
 At the moment, there are only two ways to run a file containing fostr code
-(you can find the above in `tests/emit_sum.fos`). They both start by
+(you can find the above in `tests/hw.fos`). They both start by
 cloning this fostr project. Then, either:
 
 1. Open the project in Eclipse and build it, visit your program file,
@@ -46,30 +52,70 @@ cloning this fostr project. Then, either:
    
 For example, this snippet generates the following Python:
 ```python
-{! ../tests/emit_sum.py extract:
+{! ../tests/hw.py extract:
   start: 'Stdio\s='
 !}
 ```
-(which writes "192" to standard output); it also generates identical code in
-this simple example for
-Javascript, although it generates a different preamble defining Stdio in each
-case. (Haskell code generation is also currently supported.)
+It generates nearly identical code in
+this simple example for Javascript (just with `"Hello, world!"`
+in place of `r'Hello, world!'`), although it generates a different
+preamble defining Stdio for each language. (Currently, Haskell code
+generation is also supported.)
+
+There's not much to break down in such a tiny program as this, but let's do
+it. The prefix operator `<<<` could be read as "the default stream receives...",
+and unsurprisingly in a main program the default stream is standard input and
+output. And `'Hello, world!'` is a literal string constant; what you see is
+what you get. The only detail to know is that such constants must occur
+within a single line of your source file. So depending on how you
+ran the program and how closely you looked at its output,
+you may have noticed this program does not write a newline at the end
+of its message. Nothing is ever implicitly sent to a stream. So if you want
+newlines, you should switch to a (double-quoted) string that allows
+the usual array of escape sequences:
+
+```fostr
+**/
+
+/** md */ test hello_esc_world [[
+<<< "Hello,\t\tworld!\n\n"
+]] /* **/
+parse to TopLevel(DefGets(EscString("\"Hello,\t\tworld!\n\n\"")))
+/** writes
+Hello,		world!
+
+**/
+
+/** md
+```
+(We threw in two of each so you could clearly see them in the output if
+you run this program.)
 
 ### Everything has a value
 
 As mentioned in the [Introduction](../README.md), everything in a fostr
 program (including the entire program itself) is an expression and has
-a value. So what's the value of that expression above? Well, appropriately
-enough, `stream` is our
-first example of a stream, and for convenience, the value of a stream
-receiving an item is (usually) just the stream back again. The `<<` operator
-is also left-associative, so that way we can chain insertions into a stream:
+a value. So what's the value of that expression above? Well, for convenience,
+the value of a stream receiving an item is (generally) just the stream back
+again. That way we can use the general (left-associative)
+`_stream_ << _value_` operator to chain insertions into a stream:
 ```fostr
 **/
 
-/** md */ test emit_twice [[
-stream << 72 + 87 + 33 << 291
+/** md */ test state_obvious [[
+<<< 'Two and ' << 2 << ' make ' << 2+2 << ".\n"
 ]] /* **/
+parse to TopLevel(
+  Gets(Gets(Gets(Gets(DefGets(LitString("'Two and '")),Int("2")),
+                 LitString("' make '")),Sum(Int("2"),Int("2"))),
+       EscString("\"./n\"")))
+/** writes
+Two and 2 make 4.
+**/
+
+test emit_twice [[
+stream << 72 + 87 + 33 << 291
+]]
 parse to TopLevel(
   Gets(Gets(Stream(), Sum(Sum(Int("72"), Int("87")), Int("33"))), Int("291")))
 /** writes
@@ -77,24 +123,28 @@ parse to TopLevel(
 
 /** md
 ```
-Running this program produces a nice palindromic output: "192291".
 
 And because sometimes you want to emphasize the value and propagate that
 instead of the stream, you can also write these expressions "the other way"
-with `>>`; both forms return the first argument, so the following writes "824":
+with `>>>` for sending to the default stream or `>>` in general; these forms
+(generally) return the value sent, so the following writes "824":
 ```fostr
 **/
 
 /** md */ test enters_twice [[
-(7 + 8 >> stream + 9) >> stream
+(7 + 8 >> stream + 9) >>>
 ]] /* **/
 parse to TopLevel(
-  To(Sum(Sum(Int("7"), To(Int("8"), Stream())), Int("9")), Stream()))
+  DefTo(Sum(Sum(Int("7"), To(Int("8"), Stream())), Int("9"))))
 /** writes
 824**/
 
 /** md
 ```
+Two things are worth noting here: the default stream can always be referred to
+directly via the identifier `stream`, and the precedences of `<<` and `>>` are
+different so that generally full expressions go to a stream with `<<` but
+just individual terms are sent with `>>`.
 
 ### Layout in fostr
 
@@ -104,13 +154,13 @@ lines are indented from the start of the initial line:
 **/
 
 /** md */ test receive_enter_break [[
-stream <<
+<<<
   7
-  + 8 >> stream
+  + 8 >>>
   + 9
 ]] /* **/
 parse to TopLevel(
-  Gets(Stream(), Sum(Sum(Int("7"), To(Int("8"), Stream())), Int("9"))))
+  DefGets(Sum(Sum(Int("7"), DefTo(Int("8"))), Int("9"))))
 /** writes
 824**/
 
@@ -121,8 +171,8 @@ parse to TopLevel(
 **/
 
 /** md */ test enter_receive_bad_continuation [[
-(7 + 8 >> stream + 9)
->> (stream << 9 + 2)
+(7 + 8 >>> + 9)
+>> (<<< 9 + 2)
 ]] /* **/
 parse fails
 
@@ -145,16 +195,17 @@ lines are evaluated in sequence. For example, the program
 **/
 
 /** md */ test emit_thrice [[
-  stream << 72 + 87
-  stream << 88
+  <<< 72 + 87
+  <<< 88
     + 96
-  99 + 12 >>
-          stream
+  99 + 12
+       >>>
+
 ]] /* **/
 parse to TopLevel(Sequence([
-  Gets(Stream(), Sum(Int("72"), Int("87"))),
-  Gets(Stream(), Sum(Int("88"), Int("96"))),
-  Sum(Int("99"), To(Int("12"), Stream()))
+  DefGets(Sum(Int("72"), Int("87"))),
+  DefGets(Sum(Int("88"), Int("96"))),
+  Sum(Int("99"), DefTo(Int("12")))
 ]))
 /** writes
 15918412**/
@@ -169,10 +220,10 @@ in sequence align at the left; e.g., the following fails to parse:
 **/
 
 /** md */ test emit_thrice_bad_alignment [[
-  stream << 72 + 87
-stream << 88
+  <<< 72 + 87
+<<< 88
     + 96
-  99 + 12 >> stream
+  99 + 12 >>>
 ]] /* **/
 parse fails
 
@@ -187,23 +238,23 @@ are so terminated. So the following is OK:
 **/
 
 /** md */ test emit_several [[
-  stream << 1 + 2; 3 >> stream
-  (4 + 5) >> stream; stream << 6;
-  stream << 7
-  stream << 8
+  <<< 1 + 2; 3 >>>
+  (4 + 5) >>>; stream << 6;
+  <<< 7
+  <<< 8
     + (9+10);
-  11 + 12 >> stream; 13 >> stream
-                        >> stream
+  11 + 12 >>>; 13 >>>
+                  >>>
 ]] /* **/
 parse to TopLevel(Sequence([
-  ISequence(Prior([Terminate(Gets(Stream(), Sum(Int("1"), Int("2"))))]),
-            To(Int("3"), Stream())),
-  ISequence(Prior([Terminate(To(Sum(Int("4"), Int("5")), Stream()))]),
+  ISequence(Prior([Terminate(DefGets(Sum(Int("1"), Int("2"))))]),
+            DefTo(Int("3"))),
+  ISequence(Prior([Terminate(DefTo(Sum(Int("4"), Int("5"))))]),
             Terminate(Gets(Stream(), Int("6")))),
-  Gets(Stream(), Int("7")),
-  Terminate(Gets(Stream(), Sum(Int("8"), Sum(Int("9"), Int("10"))))),
-  ISequence(Prior([Terminate(Sum(Int("11"), To(Int("12"), Stream())))]),
-            To(To(Int("13"), Stream()), Stream()))
+  DefGets(Int("7")),
+  Terminate(DefGets(Sum(Int("8"), Sum(Int("9"), Int("10"))))),
+  ISequence(Prior([Terminate(Sum(Int("11"), DefTo(Int("12"))))]),
+            DefTo(DefTo(Int("13"))))
 ]))
 /** writes
 3396727121313**/