goto is no longer restricted to appearing in tail position.

HISTORY.md

@@ -5,6 +5,8 @@ History of SixtyPical
 ----
 
 *   Syntactically, `goto` may only appear at the end of a block.
+    It need no longer be the final instruction in a routine,
+    as long as the type context is consistent at every exit.
 *   Fixed pathological memory use in the lexical scanner - should
     be much less inefficient now when parsing large source files.
 

TODO.md

@@ -69,7 +69,6 @@ error.
 If a block ends in a `call` can that be converted to end in a `goto`?  Why not?  I think it can,
 if the block is in tail position.  The constraints should iron out the same both ways.
 
-And - once we have this - why do we need `goto` to be in tail position, strictly?
 As long as the routine has consistent type context every place it exits, that should be fine.
 
 ### "Include" directives

src/sixtypical/analyzer.py

@@ -433,9 +433,6 @@ class Analyzer(object):
         dest = instr.dest
         src = instr.src
 
-        if context.encountered_gotos():
-            raise IllegalJumpError(instr, instr)
-
         if opcode == 'ld':
             if isinstance(src, IndexedRef):
                 context.assert_types_for_read_table(instr, src, dest, TYPE_BYTE)
@@ -735,6 +732,9 @@ class Analyzer(object):
         if instr.src is not None:
             context.assert_meaningful(instr.src)
 
+        if context.encountered_gotos():
+            raise IllegalJumpError(instr, instr)
+
     def analyze_for(self, instr, context):
         context.assert_meaningful(instr.dest)
         context.assert_writeable(instr.dest)

tests/SixtyPical Analysis.md

@@ -2280,7 +2280,7 @@ But only if they are bytes.
     | }
     ? TypeMismatchError
 
-A `goto` cannot appear within a `save` block, even if it is otherwise in tail position.
+A `goto` cannot appear within a `save` block.
 
     | define other routine
     |   trashes a, z, n
@@ -2325,8 +2325,7 @@ A `goto` cannot appear within a `save` block, even if it is otherwise in tail po
     | }
     = ok
 
-A `goto` cannot appear within a `with interrupts` block, even if it is
-otherwise in tail position.
+A `goto` cannot appear within a `with interrupts` block.
 
     | vector routine
     |   inputs x
@@ -2973,7 +2972,26 @@ Calling the vector does indeed trash the things the vector says it does.
     | }
     ? UnmeaningfulOutputError: x
 
-`goto`, if present, must be in tail position (the final instruction in a routine.)
+For now at least, you cannot have a `goto` inside a loop.
+
+    | define bar routine trashes x, z, n {
+    |     ld x, 200
+    | }
+    | 
+    | define main routine trashes x, z, n {
+    |     ld x, 0
+    |     repeat {
+    |         inc x
+    |         goto bar
+    |     } until z
+    | }
+    ? IllegalJumpError
+
+`goto`, as a matter of syntax, can only appear at the end
+of a block; but it need not be the final instruction in a
+routine.  It is only important that the type context at every
+`goto` is compatible with the type context at the end of
+the routine.
 
     | define bar routine trashes x, z, n {
     |     ld x, 200
@@ -2995,9 +3013,8 @@ Calling the vector does indeed trash the things the vector says it does.
     |         ld x, 1
     |         goto bar
     |     }
-    |     ld x, 0
     | }
-    ? IllegalJumpError
+    = ok
 
     | define bar routine trashes x, z, n {
     |     ld x, 200
@@ -3009,6 +3026,7 @@ Calling the vector does indeed trash the things the vector says it does.
     |         ld x, 1
     |         goto bar
     |     }
+    |     goto bar
     | }
     = ok
 
@@ -3024,7 +3042,7 @@ Calling the vector does indeed trash the things the vector says it does.
     |     }
     |     ld x, 0
     | }
-    ? IllegalJumpError
+    = ok
 
     | define bar routine trashes x, z, n {
     |     ld x, 200
@@ -3057,7 +3075,21 @@ Calling the vector does indeed trash the things the vector says it does.
     | }
     = ok
 
-For the purposes of `goto`, the end of a loop is never tail position.
+    | define bar routine trashes x, z, n {
+    |     ld x, 200
+    | }
+    | 
+    | define main routine trashes x, z, n {
+    |     ld x, 0
+    |     if z {
+    |         ld x, 1
+    |         goto bar
+    |     } else {
+    |         ld x, 0
+    |     }
+    |     ld x, 0
+    | }
+    = ok
 
     | define bar routine trashes x, z, n {
     |     ld x, 200
@@ -3065,12 +3097,15 @@ For the purposes of `goto`, the end of a loop is never tail position.
     | 
     | define main routine trashes x, z, n {
     |     ld x, 0
-    |     repeat {
-    |         inc x
+    |     if z {
+    |         ld x, 1
     |         goto bar
-    |     } until z
+    |     } else {
+    |         ld x, 0
+    |     }
+    |     goto bar
     | }
-    ? IllegalJumpError
+    = ok
 
 Can't `goto` a routine that outputs or trashes more than the current routine.