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 wrote:

On Sun, Sep 21, 2014 at 9:02 PM, Derek McLoughlin 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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