[Haskell-cafe] MTL vs Free-monads, what are your experiences

Damian Nadales damian.nadales at gmail.com
Wed Oct 19 12:29:38 UTC 2016

I was thinking, besides the evaluation of performance, the simplicity
of the approach is also important ("developer time is more expensive
than CPU time" anyone?). Note that I said simple and not easy ;)

I guess this aspect is a rather subjective one, but maybe there are
elements that can be intuitively quantified. Right now I'm playing
with free monads and MTL, to have an idea which one seems simpler to

On Mon, Oct 17, 2016 at 8:06 PM, Julian <hasufell at hasufell.de> wrote:
> On 15/10/16 15:49, Joachim Breitner wrote:
>> Hi,
>> Am Freitag, den 14.10.2016, 17:35 +0200 schrieb Damian Nadales:
>>> Do you have
>>> any experience using any of these approaches. If so would you mind
>>> sharing? ;)
>> I don’t have an answer to contribute, but I would be very interested in
>> hearing about experiences in terms of their relative runtime
>> performance.
>> My gut feeling is that an an indirect function call for every (>>=),
>> with many calls to `lift` each time, would make a deep monad
>> transformer stack much more expensive. A free monad approach seems to
>> be more sensible to me. But maybe GHC is doing a better job optimizing
>> this than I would think?
>> So if you have any number-supported evidence about this, possibly from
>> a real-world application where you tried to use one or the other,
>> please share it with us!
> There's a paper from Oleg discussing "Freer Monads, More Extensible
> Effects": http://okmij.org/ftp/Haskell/extensible/more.pdf
> The conclusion there seems to be that the EE approach is more
> "efficient". But you'll have to look at the concrete performance cases
> and data yourself to make a judgement.
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