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Today's Topics:

   1. Re:  Lazyness and forM_ (Marc Busqué)


Message: 1
Date: Fri, 13 Apr 2018 15:32:27 +0200 (CEST)
From: Marc Busqué <>
To: The Haskell-Beginners Mailing List - Discussion of primarily
        beginner-level topics related to Haskell <>
Subject: Re: [Haskell-beginners] Lazyness and forM_
Message-ID: <alpine.LNX.2.21.1804131520060.7309@localhost>
Content-Type: text/plain; charset="iso-8859-15"; Format="flowed"

On Fri, 13 Apr 2018, Kim-Ee Yeoh wrote:

> Well, 
> forM_ (categories, expenses) $ withDB . createTable
> is equivalent to 
> withDB . createTable $ expenses.
> So exactly one table is created.

Oops, you are right.

>       ```
>       ["a", "b"] `forM_` print
>       ```
>       Actually prints both `"a"` and `"b"`.
> Here the code uses square brackets--and so we have honest-to-goodness 
> lists--whereas the previous used parentheses.
> See what happens with: ("a","b") `forM_` print.

Now I feel embarrassed I overlooked that :)

I used a tuple instead of a list because in selda `Table` are type-safe.
So `categories` and `expenses` are different types and they can't go in
the same list.

So I guess, once the `forM_` mistery is gone, I can boil down my problem
to something very different. As it has nothing to do with the current
subject, I'll submit another question.

> Marc: Feel free to write to the haskell-cafe mailing list for questions such 
> as this. Fortuitously in this case, it turns out that your query
> needed knowledge only about the forM* combinators and--ever since their ilk 
> was generalized--the known instances declared for the Traversable
> constraint. As you explore the domain-specific package "selda" further, you 
> will find more people acquainted with the package over at the cafe
> than here in beginners. Suffice to say, everyone here in this list is also in 
> cafe, which is also open to beginners questions.

And following your recommendation I'll submit that question in the
haskell-cafe mailing list :)

Thank you very much for your help :)

Marc Busqué


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