[Haskell-cafe] streaming translation using monads

Justin Bailey jgbailey at gmail.com
Wed Nov 19 15:19:17 EST 2008

On Wed, Nov 19, 2008 at 11:50 AM, Warren Harris
<warrensomebody at gmail.com> wrote:
> 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?

If the type of the input determines the type of the output, and the
type of the input can be determined statically, then I think you are
overlooking this technique. But if your application requires that each
input expression be sent to the remote client before type information
can be determined, then you are right.

In the case of HaskellDB, each operator/term in the Query monad
enriches the type information available. Of course, that enrichment
happens at compile time and occurs via type inference. For example,
assuming two tables T1 and T2, with columns T1col and T2col, this

  simpleQuery = do
     t1 <- table T1
     t2 <- table T2
     restrict ( .. some expression ...)
     project (t1 ! T1col1 # t2 ! T2col1)

Gives a type similar to Query (RecCons T1Col1 Int (RecCons T2 Col2 Int
RecNil)). RecCons and RecNil are types that allow a list to be built
at the type level. The list carries the column names and types in the
resulting projection. The type information is used  to ensure queries
only refer to columns that exist, datatypes are compared sensibly,
etc. If in your case you can build that kind of structure based purely
on the "input language" operators/terms, then it seems you could build
the entire "output expression" in one shot. But again, if input and
output have to be interleaved then I think you are stuck.


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