[Haskell-beginners] Sqlite3 - INSERT statement question

[Patrick Lynch]

...looks good, thank you - see following: [note: the '1' on the 2nd line 
indicates a successful command execution] -- 

Prelude Database.HDBC Database.HDBC.Sqlite3> run conn "INSERT INTO test 
VALUES (?, ?)" [toSql (0::Int), toSql ("zero"::[Char])]
1

----- Original Message ----- 
From: "Brent Yorgey" <byorgey at seas.upenn.edu>
To: <beginners at haskell.org>
Sent: Thursday, February 03, 2011 12:25 PM
Subject: Re: [Haskell-beginners] Sqlite3 - INSERT statement question


> On Thu, Feb 03, 2011 at 12:20:12PM -0500, Patrick Lynch wrote:
>> Good morning,
>>
>> I'm using "Real World Haskell" and a Windows Vista PC and I have Sqlite3 
>> installed...[note: I had to change test1.db to /users/user/test1.db in 
>> order to get this to work, otherwise, neither the database nor the table 
>> could be created -- also, note c:/users/user/test1.db gives a syntax 
>> error], ghci doesn't like c:/]:
>>
>> I tried to use the following example from the book but it failed...see 
>> following:
>>
>> Prelude> :m Database.HDBC Database.HDBC.Sqlite3
>> Prelude Database.HDBC Database.HDBC.Sqlite3> conn <- connectSqlite3 
>> "/users/user/test1.db"
>> ...
>> Prelude Database.HDBC Database.HDBC.Sqlite3> run conn "INSERT INTO test 
>> VALUES (?, ?)" [toSql 0, toSql "zero"]
>>
>> <interactive>:1:43:
>>     No instance for (Data.Convertible.Base.Convertible t SqlValue)
>>       arising from a use of `toSql' at <interactive>:1:43-49
>>     Possible fix:
>>       add an instance declaration for
>>       (Data.Convertible.Base.Convertible t SqlValue)
>>     In the expression: toSql 0
>>     In the third argument of `run', namely `[toSql 0, toSql "zero"]'
>>     In the expression:
>>         run conn "INSERT INTO test VALUES (?, ?)" [toSql 0, toSql
>>     "zero"]
>
> I think the problem is that 0 is polymorphic, so GHC does not know
> what type you want it to have, but its type determines which
> Convertible instance is chosen.  It should work if you put a type
> signature on the 0, like
>
>  toSql (0 :: Int)
>
> or
>
>  toSql (0 :: Integer)
>
> or whatever.
>
> -Brent
>
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