On 27/07/2012 15:18, Simon Fowler wrote:
Dear all,
I'm currently working on a project which would benefit from access to
the parse tree - ideally, we would like to deconstruct an expression
into its constituent types. Currently we are using the exprType function
to return the type of an expression, but this is limited as it only
works for one 'level', per se.
An example would be as follows. Currently, if we were to type in "map",
we would be given a result of type Type, which we could further
deconstruct into the various subtypes, as so:
map :: (a -> b) -> [a] -> [b]
split into:
(a -> b)
[a]
[b]
This is sufficient for the basics of the project at the moment, but
ideally, we would like to use the parse tree to analyse the structure of
expressions and thereby the types of the corresponding sub-expressions.
Take "foldr drop" for example; we can determine the different types for
the different functions with exprType:
foldr :: (a -> b -> b) -> b -> [a] -> [b]
drop :: Int -> [c] -> [c]
Now we can call exprType on the application "foldr drop":
foldr drop :: (Int -> [c] -> [c]) -> [c] -> [Int] -> [c]
which would be split, according to our current code, into:
arg1: (Int -> [c] -> [c])
arg2: [c]
arg3: [Int]
result: [c]
The problem here is that we are unable to separate the "drop" from the
"foldr". The project we are working on involves composing and
decomposing expressions, and it is important that we can decompose the
type of "foldr drop" into the types of the sub-expressions "foldr" and
"drop" recursively.
Ideally, we would like to construct a data structure which is much more
akin to a parse tree with type annotations, in this case:
PTreeApp
(PTreeExpr "foldr" [| (a -> b -> b) -> b -> [a] -> [b] |])
(PTreeExpr "drop" [| Int -> [c] -> [c] |])
[| (Int -> [c] -> [c]) -> [c] -> [Int] -> [c] |]
where the types in semantic brackets are a structural representation
(e.g. TypeRep.Type) of the given types.
Looking at the code of exprType, a call is firstly made to hscTcExpr,
which in turn makes a call to hscParseStmt to return the parse tree.
This would seem to provide the functionality that we would require, in
that it would give access to a type-checkable parsed statement, but it
doesn't seem to be exported by HscMain. Is there another function, which
is accessible through the API, that would support this or something similar?
I am far from an expert using the GHC API, so apologies if I am doing
something grossly wrong or have missed something blatantly obvious.
What you want is unfortunately not provided by the GHC API at the
moment. exprType only returns the Type; I think what you want is the
typechecked abstract syntax for the expression, so that you can traverse
and analyse it.
It wouldn't be hard to do this. TcRnDriver.tcRnExpr needs to return the
LHsExpr Id (maybe it needs to be zonked first), and then we need to
expose an API to provide access to this via the GHC module. Feel free
to have a go yourself, or make a ticket and we'll hopefully get around
to it.
Cheers,
Simon
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