Re: access numeric data in module - Mailing list pgsql-hackers

Tom-
    I understand that you are concerned about future maintenance costs vs benefits of this change. I hope that I can address those concerns. An important thing to note is that there are two different structures that represent numeric values:
    * NumericData is an opaque structure that is defined in numeric.c. It is this struct that is used to store values. The patch I submitted has this structure remain opaque and in numeric.c. Its internals are messy and subject to future changes. I agree that third parties should not have access to this. Of note is that the type Numeric is a typedef of NumericData*.
    * NumericVar is a user-friendly structure that already exists. It is this structure that I propose moving to numeric.h. There are functions that exist to convert it to and from NumericData. It is these functions that I propose giving access to.

    What the patch boils down to is the movement of NumericVar to numeric.h along with function declarations for the basic function to work with it and a few pre-processor declarations.

    I agree that there is the potential for future maintenance costs here. However, any future changes to NumericData would necessitate updating the code to convert to and from NumericVar regardless of the proposed changes. I think that this small increase in costs is outweighed by the benefits of allowing third parties to access this powerful datatype.

    As for the reason that I would like to make this change: I am the maintainer of the PL/Haskell extension. (It allows the use of Haskell code as a procedural language. https://github.com/ed-o-saurus/PLHaskell) In the extension, users can currently pass several Postgres types and also have the function return them. This is accomplished by using the functions and macros that convert between Datums and C data types. (For example DatumGetFloat8 and Float8GetDatum to handle double-precision floating point values) I would like to add support for the use of the numeric type to the extension. To this end, I would need to create a Haskell type that mirrors the Postgres numeric type. Passed Haskell values would be instantiated by reading the data from Postgres. Conversely, returned values would be converted to the Postgres type. Internally, users would be able to perform mathematical operations with the Haskell values like any other type. Currently, there is no way for a third-party extension to get needed information about numeric values or build new numeric values. The proposed changes would remedy this.

    An alternative approach would be to make available calls to read and create numeric data. However, as the NumericVar struct already exists, I feel that utilizing it is the more natural approach.

    What do you think?
              -Ed