Access to the Parse Tree for Expressions

[Simon Fowler]

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.

Thank you for taking the time to read this.

Kind regards,
Simon
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