Using a monadic operation in a list comprehension.
Hi, I'm trying to get Graham Hutton's "Programming with Effects" article (http://www.cs.nott.ac.uk/~gmh/monads) working with Data.Tree instead of the Tree he provides. My code is here: https://gist.github.com/derekmcloughlin/95ab690e0c07c5a7221e The compiler is complaining about the use of lab <- mlabel c in the list comprehension: Couldn't match expected type ‘[Tree (a, Int)]’ with actual type ‘ST (Tree (a, Int))’ This makes sense, but is there any way to execute the monad operation "mlabel" within the list comprehension? I've tried the MonadComprehensions extension but still get an error. Derek.
On Sun, Sep 21, 2014 at 9:02 PM, Derek McLoughlin < derek.mcloughlin@gmail.com> wrote:
I'm trying to get Graham Hutton's "Programming with Effects" article (http://www.cs.nott.ac.uk/~gmh/monads) working with Data.Tree instead of the Tree he provides.
Hi Derek, This is a lovely problem that's inspired a bunch of interesting papers recently. Here's a stab at the compilation error you're facing: http://www.atamo.com/blog/how-to-solve-a-tricky-monad-problem-1/ Enjoy :) -- Kim-Ee
Thanks very much for your clear explanation! Worked perfectly. You're right about the ST - I got very confused at the start with Control.Monad.ST. I'll be removing Graham's ST in favour of the 'normal' State monad. On 21 September 2014 19:38, Kim-Ee Yeoh <ky3@atamo.com> wrote:
On Sun, Sep 21, 2014 at 9:02 PM, Derek McLoughlin <derek.mcloughlin@gmail.com> wrote:
I'm trying to get Graham Hutton's "Programming with Effects" article (http://www.cs.nott.ac.uk/~gmh/monads) working with Data.Tree instead of the Tree he provides.
Hi Derek,
This is a lovely problem that's inspired a bunch of interesting papers recently.
Here's a stab at the compilation error you're facing:
http://www.atamo.com/blog/how-to-solve-a-tricky-monad-problem-1/
Enjoy :)
-- Kim-Ee
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On Mon, Sep 22, 2014 at 3:55 AM, Derek McLoughlin < derek.mcloughlin@gmail.com> wrote:
Thanks very much for your clear explanation! Worked perfectly.
Thanks for taking the time to say this :) I hope I didn't belabor the mismatched effects bit. Combining effects is a wide open field of research if you look at papers like Extensible Effects, Monad Coproducts, etc. Point being, there just might be a way to make your code as originally written compile and work as you expect it to. Ah, the joys of life at the bleeding edge! -- Kim-Ee
participants (2)
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Derek McLoughlin -
Kim-Ee Yeoh