[web-devel] Esqueleto type trickery question
gael.deest at gmail.com
Sat Nov 24 12:35:51 CET 2012
Well, there obviously is a problem in your example code. The argument
"tableName" does not live in the world of types, so /SqlExpr (Entity
tableName)/ is meaningless (this barrier is getting fuzzier with each
version of GHC, but well...).
Perhaps something can be done with existential types ?
Le 24/11/12 12:07, Erik de Castro Lopo a écrit :
> Hi all,
> Below my .sig is a little Persist/Esqueleto program that works correctly.
> The getUserCount function does thr right thing and returns the number of
> rows in the User table. However, what I'd like is a generic function
> that returns the row count of any table, something vaguely like this:
> queryRowCount :: tableType -> SqlPersist IO Int64
> queryRowCount tableName = do
> [Value x] <- select . from $ \(_ :: SqlExpr (Entity tableName)) ->
> return countRows
> return x
> Is there any way to do this? Its probably possible in Agda, but can it
> be made to work with GHC?
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