Hi there! I'm using [selda](https://github.com/valderman/selda) package to work with databases. I'm trying to write a function to generate several tables at once. In the following examples, `categories` and `expenses` are two selda `Table`. The other functions in use and their respective imports should be self-evident: If I do: ``` migrate :: IO () migrate = do dir <- dBDir createDirectoryIfMissing True dir forM_ (categories, expenses) $ withDB . createTable ``` tables are not actually created. However, if I do: ``` migrate :: IO () migrate = do dir <- dBDir createDirectoryIfMissing True dir withDB . createTable $ categories withDB . createTable $ expenses ``` Both tables are actually created. So it seems like the thugs created with `formM_` are actually never executed. Is it so? I'm new with Haskell and it seems strange for me. If it is so, doesn't make it useless `forM_` for IO? Furthermore, I can't reproduce it in ghci. Doing this: ``` ["a", "b"] `forM_` print ``` Actually prints both `"a"` and `"b"`. Thanks for any enlightment. Marc Busqué http://waiting-for-dev.github.io/about/
Hi Marc, and welcome to the haskell beginners list. ```
migrate :: IO () migrate = do dir <- dBDir createDirectoryIfMissing True dir forM_ (categories, expenses) $ withDB . createTable ```
tables are not actually created.
Well, forM_ (categories, expenses) $ withDB . createTable is equivalent to withDB . createTable $ expenses. So exactly one table is created.
``` ["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. 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. p.s. Veterans would recognize this as ye olde controversy on the Foldable instance for pairs introduced in GHC 7.10.x. The controversy still simmers apparently because there isn't an instance for triples and higher tuples in the latest and greatest GHC 8.4.1. We are at the mercy of this potentially toe-stubbing absence of uniformity. -- Kim-Ee
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 http://waiting-for-dev.github.io/about/
participants (2)
-
Kim-Ee Yeoh -
Marc Busqué