This comes from this issue in the Perl 6 documentation, on the differential treatment of colonpairs; while :a is equivalent to a => True, (:a:b) is (a => True b => 1) and :a:b or :a :b simply drops the second; colonpairs behave in a different way depending on where they are in the second or first position.
So I have looked at the QAST, which prints this:
- QAST::Block(:cuid(2)) :in_stmt_mod<?> #!/usr/bin/env perl6\n\nuse v6;\n\nsay (:a:b);\n
- QAST::Var(local __args__ :decl(param))
- QAST::Stmts #!/usr/bin/env perl6\n\nuse v6;\n\nsay (:a:b);\n
- QAST::Op(call)
- QAST::Block(:cuid(1) :blocktype(declaration_static)) :outer<?> :in_stmt_mod<?> :code_object<?> :IN_DECL<mainline>
- QAST::Stmts #!/usr/bin/env perl6\n\nuse v6;\n\nsay (:a:b);\n
- QAST::Var(lexical $ยข :decl(contvar))
- QAST::Var(lexical $! :decl(contvar))
- QAST::Var(lexical $/ :decl(contvar))
- QAST::Var(lexical $_ :decl(contvar))
- QAST::Var(lexical GLOBALish :decl(static))
- QAST::Var(lexical EXPORT :decl(static))
- QAST::Var(lexical $?PACKAGE :decl(static))
- QAST::Var(lexical ::?PACKAGE :decl(static))
- QAST::Var(lexical $=finish :decl(static))
- QAST::Var(lexical $=pod :decl(static))
[value]
-
- QAST::Var(lexical !UNIT_MARKER :decl(static))
- QAST::Stmts
- QAST::Op(bind)
- QAST::Var(local ctxsave :decl(var))
- QAST::Var(contextual $*CTXSAVE)
- QAST::Op(unless)
- QAST::Op(isnull)
- QAST::Var(local ctxsave)
- QAST::Op(if)
- QAST::Op(can)
- QAST::Var(local ctxsave)
- QAST::SVal(ctxsave)
- QAST::Op(callmethod ctxsave)
- QAST::Var(local ctxsave)
- QAST::Stmts
- QAST::WVal(Array)
- QAST::Stmts <sunk> ;\n\nsay (:a:b);\n
- QAST::Stmt <sunk final> say (:a:b)
- QAST::Want <sunk>
- QAST::Op(call &say) <sunk> :statement_id<2> say (:a:b)
- QAST::Stmts <wanted> (:a:b)
- QAST::Op(call &infix:<,>)
- QAST::Op(callmethod new) <wanted> :a
- QAST::Var(lexical Pair) <wanted> :a
- QAST::Want <wanted>
- QAST::WVal(Str)
- Ss
- QAST::SVal(a)
- QAST::Op(hllbool) <wanted>
- QAST::IVal(1)
- QAST::Op(callmethod new) <wanted nosink> :b
- QAST::Var(lexical Pair) <wanted> :b
- QAST::Want <wanted>
- QAST::WVal(Str)
- Ss
- QAST::SVal(b)
- QAST::Want <wanted>
- QAST::IVal(1)
- v
- QAST::Op(p6sink)
- QAST::Op(call &say) <sunk> :statement_id<2> say (:a:b)
- QAST::Stmts <wanted> (:a:b)
- QAST::Op(call &infix:<,>)
- QAST::Op(callmethod new) <wanted> :a
- QAST::Var(lexical Pair) <wanted> :a
- QAST::Want <wanted>
- QAST::WVal(Str)
- Ss
- QAST::SVal(a)
- QAST::Op(hllbool) <wanted>
- QAST::IVal(1)
- QAST::Op(callmethod new) <wanted nosink> :b
- QAST::Var(lexical Pair) <wanted> :b
- QAST::Want <wanted>
- QAST::WVal(Str)
- Ss
- QAST::SVal(b)
- QAST::Want <wanted>
- QAST::IVal(1)
- QAST::WVal(Nil)
The main difference is that the first colonpair is - QAST::Op(callmethod new) <wanted> :a, while the second is - QAST::Op(callmethod new) <wanted nosink> :b, please note the nosink. The, I guess, important part is where an implicit operator , is called; this operator sinks the first element. But I wonder why the effect of sinking is making the value in the Pair equal to True, while not sinking it makes it equal to 1. Is that intended? Some side effect or something I can't really understand? None of the above?
