[Haskell-cafe] Specializing argument to forM_, mapM_ & foldM_ in base-4.8

Amit Aryeh Levy amit at amitlevy.com
Thu Oct 15 17:16:48 UTC 2015

I've been running into a relatively small but frequent annoyance with
base >= 4.8 (GHC 7.10). `Control.Monad.foldM_`, `Control.Monad.mapM_`
and `Control.Monad.forM_` are generalized traverse over any `Foldable a`
rather than just arrays (`[a]`).

 This is great, except I'm finding that, for a lot of my code that works
well in previous versions, I need to specialize the argument to `[a]`
now. If other people are encoutering a similar patter, I wonder what are
your best practices for doing this: ScopedTypeVariables? Deconstruct the
reconstruct the array? ...

 The most common example is when I deserialize a JSON array with aeson
and want to traverse over that array (say, to store the objects to a DB):

let objArray = eitherDecode myjson
case objArray of
    Left err -> ...
    Right (objs :: [MyObjType]) ->
        forM_ objs $ \obj -> saveToDb obj

​The above fix requires `ScopedTypeVariables` (which is probably OK).
Another option is to deconstruct and reconstruct the list:

Right (o:objs) ->
    forM_ (o:objs) $ \obj -> saveToDb obj

Does this get optimized away?

Penny for your thoughts?


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