Re: [Haskelldb-users] errors evaluating aggregates

[Justin B. <jgb...@gm...>]

I think your problem may be the database driver, not haskelldb. What
database are you talking to? Can you get any more useful output (try
running with "+RTS -xc" option). From the GHC documentation:

"-xc

(Only available when the program is compiled for profiling.) When an
exception is raised in the program, this option causes the current
cost-centre-stack to be dumped to stderr.

This can be particularly useful for debugging: if your program is
complaining about a head [] error and you haven't got a clue which bit
of code is causing it, compiling with -prof -auto-all and running with
+RTS -xc -RTS will tell you exactly the call stack at the point the
error was raised."

Justin

On Wed, Jun 4, 2008 at 6:10 PM, "Rafael J. Fernández-Moctezuma"
<rfe...@cs...> wrote:
> Hello everybody!
>
> I am getting a runtime error when I evaluate aggregates. For example,
> consider:
>
> query1 = do s <- table sailors
>            project (sid <<  _sum (s!sid))
>
> "sid" is an attribute in table "sailors", of type Int (and NOT NULL).
> When evaluating, I get a runtime error:
>
> user error (The type of sid field can't be converted to SqlUnknown 246 type)
>
> My intuition is that the type of a sum expression is not necessarily
> Int, since you can always sum Doubles. Well, the next obvious step for
> me was to rewrite the query and make the "receiving" column type a double:
>
> data Doublefield = Doublefield
>
> instance FieldTag Doublefield where fieldName _ = "doublefield"
>
> doublefield ::  Attr Doublefield Double
> doublefield = mkAttr Doublefield
>
> query2 = do s <- table sailors
>            project (doublefield <<  _sum (s!sid))
>
>
> Oops, that won't compile, the inferred type of sum is indeed Int (or so
> I'm told by GHC):
>
>    Couldn't match expected type `Double' against inferred type `Int'
>      Expected type: Expr Double
>      Inferred type: Expr Int
>
> Strangely, count, max, and min do work. I wonder if (1) I'm missing
> something, or (2) this is indeed an unexpected behaviour (given query1
> passes the typechecker) that may be specific to MySQL...
>
> Thanks,
>
> RJFM
>
>
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