[Haskell-cafe] QuickCheck testing of AST transformers

Lennart Augustsson lennart at augustsson.net
Mon Apr 23 18:05:16 EDT 2007

Without looking into your language and transformation in more detail  
it's hard to come up with concrete suggestions.  But here are some  

Write an interpreter for each of your languages (original AST,  
transformed AST) etc, and then use a quickcheck property stating that  
well formed programs have the same denotation before and after  
transformation, i.e., the two interpreters give the "same" value (you  
might need some relaxed notion of same).

You transformations are trying to get rid of some language construct,  
I presume.  So you can have some properties stating that they will be  
gone in the transformed program..

	-- Lennart

On Apr 23, 2007, at 22:46 , Joel Reymont wrote:

> 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
> --
> http://wagerlabs.com/
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