Unfortunately there appears to be exactly one place you can do this, and it
turns out to be inside inferTypes (because you need to know the type of c
completely, and you can't type-infer x until you know if it's a tag or a
variable). It's definitely nicer than doing it in the parser, though.
I've also started adding "-- AMBIGUITY" comments in the parser.
This makes sure that literals produced by the constant evaluator will never
contain UnknownDimension. The change looks a lot more complex than it really
is; it already carried the type "downwards", and most of this is just making it
carry it back up to where the A.Literal is being constructured.
This is looking at array dimensions in literals. By this point unknown
dimensions should have been inferred; if they haven't, I'd rather it blew up
here than caused bizarre behaviour later on.
(This is a good candidate for a property check.)
This means you don't have to have folded constants throughout the tree/state in
order to evaluate them, which makes the early passes less awkward to manage.
Type inference and checking is now handled entirely by the later passes.
There are a few remaining places in the parser that look at the types of things
that have been defined; this is in order to resolve syntax ambiguities (e.g.
c[x], c ! x). This is a temporary measure to minimise cgtest breakage; it can't
work for things that need to be inferred (e.g. "CHAN INT c:" "d IS c:" "d !
x"), so it'll need moving out to a pass in the near future.
There's still quite a bit of work to do on this, but results so far are
encouraging: the code is an awful lot cleaner, and about four hundred lines
shorter.
This infers the types of literals and abbreviations.
This is not yet complete, but it's mostly there. I was surprised at how complex
it turned out to be, but it's significantly less awkward than having it
threaded through the parser (plus it works correctly, unlike the old code).
There are a few FIXMEs for things I've yet to implement.
The rendering code now takes the type it's aiming for, so it can produce a
value of exactly the same type as the expression that was being folded
originally (rather than trying to work it out for itself).
Subscripting Infer gives you Infer.
This also makes trivialSubscriptType handle user types in the same way as
subscriptType. (There probably isn't much reason to have both any more, since
subscriptType doesn't do the checks it used to do.)
Previously the parser set it to the element type, but everything that used it
set it to the type of the whole array. Now it's documented in AST.hs, and the
parser makes it the type of the whole array, since that's almost always what
you really want later on.
It now just uses "words" and regular pattern matches rather than regular
expressions. The resulting code is quite a bit simpler, and goes much faster.
I've added some unit tests for it too.
This is solely because GHC 6.6 doesn't like them (it complains about the type
variable already being in scope -- which it is, but there's nothing I can do
about that!). This doesn't lose any safety; if you try to write a
transformation for something that's not Data you'll find out when you try to
pass it to one of the application functions.
We now have three kinds of canned tree traversals, all of which are smart about
which types they're applied to: explicit-descent transformations,
implicit-descent transformations, and implicit-descent checks. I've only
provided depth-first application of the latter two, but we could do
breadth-first in the future if necessary.
