[Haskell-cafe] Re: The danger of Monad ((->) r)

[Arie Peterson]

Hi Tomek!


> In a CGI application I was gathering SQL statements I wanted to run in
> the final transaction. Because I use haskelldb, it was most convenient
> to use (Database -> IO ()) as the type of the statement or a group of
> statements. In this representation concatenating two statement groups so
> they are executed in sequence can be done with:
>
>     concatStmts s1 s2 = \db -> s1 db >> s2 db
>
> My mistake was that I forgot about db and wrote:
>
>     concatStmts s1 s2 = s1 >> s2
>
> And it was accepted because I had the Monad instance for ((->) r) in
> scope (from Control.Monad.Trans I guess)!

Have you considered changing the statements to have type 'ReaderT Database
IO ()'? Then (>>) actually does what you want.


Pozdrawiam,

Arie


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