[Haskell-cafe] QuickCheck testing of AST transformers
joelr1 at gmail.com
Mon Apr 23 17:46:32 EDT 2007
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 .
Array: MyArray(10 + 2);
Value1 = MyArray;
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
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")
, 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
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