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
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
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