Hi
My experience is that generating correct AST's is hard to get right.
I've found regression testing to be more useful when doing program
transformation.
I would follow Lennarts solution of writing a function that
semantically evaluates each expression. I would then add a bit into
your program which automatically does this check after each
transformation. This way if your code goes wrong you'll find out
sooner.
Thanks
Neil
On 4/23/07, Joel Reymont
My previous post did not receive any replies so I thought I might try generalizing the problem a bit...
Suppose I'm parsing a language into a syntax tree and then transforming that tree into another AST representing a "core language". The core language is a more general AST that should help with compiling to other languages.
My problem is how to best structure my AST transformations to be able to test them with QuickCheck. I suspect that I'm not going about it in the most optimal way so I thought I should ask for suggestions.
The transformation into the core AST applies operations to simplify, or desugar, the AST of the original language. Here's sample code in the source language which, incidentally, was recently highlighted at Lambda the Ultimate [1].
Array: MyArray[10](10 + 2); Value1 = MyArray[5][10];
This declares an array of 10 elements and initializes each element to 12. Value1 (a built-in variable) is then initialized to the value of element #5 as of 10 bars ago. A bar is, basically, a stock quote. The code is invoked on every bar and so "5 bars ago" can be treated as 5 invocations ago.
The syntax tree of the above code is a 1-1 mapping. We declare an array of integers of 10 elements. Initialize it to the sum of two integers and then assign to Value1.
[ ArrayDecs [ VarDecl (VarIdent "MyArray") TyInt [Int 10] (Op Plus (Int 10) (Int 2)) ] , Assign (VarIdent "Value1") [] (Var (VarIdent "MyArray") [Int 5] (BarsBack (Int 10))) ]
The "desugared" version does away with the array declaration statement and declares MyArray to be a variable of array type. Arrays in the "core language" do not remember values from one invocation to another but there's a data series type, so we declare a series variable to hold the value of element #5.
We must manually store the value of the array element in the data series and can then refer to the value of the series 10 data points ago.
vars = [ ("MyArray", VarDecl (TyArray TyInt) [Int 10] (Just (Plus (Int 10) (Int 2)))) , ("series0", VarDecl (TySeries TyInt) [] Nothing) ]
code = [ AddToSeries (VarIdent "series0") (Var (VarIdent "MyArray") [Int 5]) , Assign (Var (VarIdent "Value1") []) (Series (VarIdent "series0") (Int 10)) ]
The next step would be to take the above "core syntax tree" and transform it yet again into a C# (or other target language) AST. It's assumed that all target languages have a data series type.
The OCaml version of my code translated directly into the C# AST but I figured an intermediate syntax tree will help me translate into other languages such as Haskell, Erlang or OCaml.
The part I can't figure out is how to come up with a set of invariants for my transformations.
Should I, for example, state that every access to an array value in a previous invocation should introduce an extra variable to hold the series plus the appropriate assignment code?
Should I write the translator as a series of small transformers in the ST monad that can be threaded and tested separately?
Thanks in advance, Joel
[1] http://lambda-the-ultimate.org/node/2201
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