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lpsmith / postgresql-simple

Mid-level client library for accessing PostgreSQL from Haskell
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_int8 treated as incompatible with Int64, Integer, and Int executing a query - Stack Nightly 64 bit Linux #190

Closed autcrock closed 8 years ago

autcrock commented 8 years ago

Hi Leon.

I have reached the limit of my out-of-date Haskell debugging skills with this one, so reaching out for help.

http://stackoverflow.com/questions/38735008/haskell-postgresql-simple-incompatible-type-int8-and-int64-and-integer

Hopefully it is nothing more than a misapprehension of the documentation on my part.

Cheers

Mike Thomas

lpsmith commented 8 years ago

Sure, the underscore in _int8 means it's an array of 8-byte integers. So you need to change the return type to [Only (Vector Int64)]

lpsmith commented 8 years ago

Or, don't call _array_agg, which would probably be a somewhat better choice in this situation.

autcrock commented 8 years ago

Thanks Leon, not only for the answer which worked, but for your work on postgresql-simple which I am finding very helpful.

The _array_agg will go away soon. It was a by product of my initial attempts to get the marshalling types correct.

lpsmith commented 8 years ago

You are welcome!

lpsmith commented 8 years ago

Looking at the Database.PostgreSQL.Simple.FromField documentation, this isn't mentioned. So that is worth fixing.

Unfortunately the underscore meaning "array of" is a convention; I don't think it's a hard and fast rule. I know that some of the older built in types have an associated array type that doesn't start with an underscore. It's probably possible to create a non-array type that starts with an underscore as well. The more robust approach is checking the pg_type table, which contains metadata for every type (built in or not)

Try this query in psql: select oid, * from pg_type; the partial contents of this metatable is also accessible through postgresql-simple's TypeInfo system.

autcrock commented 8 years ago

Thanks for that further information Leon.

I was the victim of ten years of C programming seeing the _ as an indicator of an internal type, managing to convince myself it was on the Haskell side, although I didn't see it as I skimmed through the postgres-simple source code.

Cheers

Mike Thomas

On Fri, Aug 5, 2016 at 10:54 PM, Leon P Smith notifications@github.com wrote:

Looking at the Database.PostgreSQL.Simple.FromField documentation, this isn't mentioned. So that is worth fixing.

Unfortunately the underscore meaning "array of" is a convention; I don't think it's a hard and fast rule. I know that some of the older built in types have an associated array type that doesn't start with an underscore. It's probably possible to create a non-array type that starts with an underscore as well. The more robust approach is checking the pg_type table, which contains metadata for every type (built in or not)

Try this query in psql: select oid, * from pg_type; the partial contents of this metatable is also accessible through postgresql-simple's TypeInfo https://hackage.haskell.org/package/postgresql-simple-0.5.2.1/docs/Database-PostgreSQL-Simple-TypeInfo.html system.

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