[Haskell-cafe] streaming translation using monads

Warren Harris warrensomebody at gmail.com
Wed Nov 19 14:50:13 EST 2008

On Nov 19, 2008, at 9:11 AM, Justin Bailey wrote:

> On Tue, Nov 18, 2008 at 6:23 PM, Warren Harris <warrensomebody at gmail.com 
> > wrote:
>> I am working on a query language translator, and although I feel  
>> that a
>> monadic formulation would work well for this application, I've  
>> stumbled on a
>> number of questions and difficulties that I thought the  
>> knowledgeable people
>> here might be able to help me with.
> HaskellDB takes a similar approach. It's "Query" monad allows you to
> build queries which are then translated to SQL by a "runQuery"
> function. Could your bind operation collect the 'input' expressions
> and then output them all at once via a "runTranslation" function?

Thanks for pointing this out. I had looked at Leigen & Meijer's paper  
a while back which describes this, and in one branch of my code taken  
a very similar approach by using an abstract datatype with phantom  
types to represent the target language. However, I had concluded  
(perhaps incorrectly) that the technique was not amenable to streaming  
the results back to the client without first constructing an in-memory  
list with a universal type representing the values returned.

Now perhaps the in-memory list part was a bad conclusion since the  
queries can be decorated with translation functions capable of  
streaming the results out to another channel. However, the use of a  
universal type for the values would still seem to be required since  
there is no way to implement type-indexed values when the queries  
themselves are expressed as an abstract datatype rather than as  
functions. Am I overlooking something?

As an aside, maybe I've been unduly concerned with wrapping the raw  
values in a universal type since I've found that in most cases for my  
target language I must deal with the possibility of null results, and  
must wrap most values in a Maybe type anyway. So I must pay the  
allocation cost in most cases... and with phantom types perhaps I can  
avoid the possibility of dynamic type errors due to unexpected data or  
ill-formed target queries.

> Do
> you have to do in-place translation?

Not sure I follow.


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