hclsyntax: Initial work on namespaced functions

This introduces a new syntax which allows function names to have namespace
prefixes, with the different name parts separated by a double-colon "::"
as is common in various other C-derived languages which need to
distinguish between scope resolution and attribute/field traversal.

Because HCL has separate namespaces for functions and variables, we need
to use different punctuation for each to avoid creating parsing ambiguity
that could be resolved only with infinite lookahead.

We cannot retroactively change the representation of function names to be
a slice of names without breaking the existing API, and so we instead
adopt a convention of packing the multi-part names into single strings
which the parser guarantees will always be a series of valid identifiers
separated by the literal "::" sequence. That means that applications will
make namespaced functions available in the EvalContext by naming them in
a way that matches this convention.

This is still a subtle compatibility break for any implementation of the
syntax-agnostic HCL API against another syntax, because it may now
encounter function names in the function table that are not entirely
valid identifiers. However, that's okay in practice because a calling
application is always in full control of both which syntaxes it supports
and which functions it places in the function table, and so an application
using some other syntax can simply avoid using namespaced functions until
that syntax is updated to understand the new convention.

This initial commit only includes the basic functionality and does not yet
update the specification or specification test suite. It also has only
minimal unit tests of the parser and evaluator. Before finalizing this
in a release we would need to complete that work to make sure everything
is consistent and that we have sufficient regression tests for this new
capability.

1 files changed, 3 insertions, 2 deletions

diff --git a/hclsyntax/token.go b/hclsyntax/token.go
index afde5f3..47648b8 100644
--- a/hclsyntax/token.go
+++ b/hclsyntax/token.go
@@ -63,8 +63,9 @@ const (
 	TokenDot   TokenType = '.'
 	TokenComma TokenType = ','
 
-	TokenEllipsis TokenType = '…'
-	TokenFatArrow TokenType = '⇒'
+	TokenDoubleColon TokenType = '⸬'
+	TokenEllipsis    TokenType = '…'
+	TokenFatArrow    TokenType = '⇒'
 
 	TokenQuestion TokenType = '?'
 	TokenColon    TokenType = ':'