If you care about performance you may - I haven't benchmarked - want to use Vector instead of lists here since that's what aeson uses internally. Then it's pretty handy that you can still use forM_.

It's possible that the list pattern deconstruction and list construction gets optimized away, my gut says you need -O2 for that to happen. Here's a good explanation on how to dump and read core so you can check for yourself what happens in this case: http://stackoverflow.com/questions/6121146/reading-ghc-core . Either way it's definitiely not less efficient to annotate the type instead. You don't need ScopedTypeVariables here, you can write the type inside an expression instead: `forM (objs :: Type) [...]`

HTH,

Adam

On Thu, Oct 15, 2015 at 7:16 PM, Amit Aryeh Levy <amit@amitlevy.com> wrote:

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?

Cheers!
Amit

_______________________________________________
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe