[Haskell-cafe] QuickCheck testing of AST transformers

Neil Mitchell ndmitchell at gmail.com
Mon Apr 23 18:52:47 EDT 2007


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

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



On 4/23/07, Joel Reymont <joelr1 at gmail.com> 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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