[Haskell-cafe] Abuse of the monad

Thomas Davie tom.davie at gmail.com
Thu Mar 12 10:37:40 EDT 2009

On 12 Mar 2009, at 15:33, Colin Paul Adams wrote:

>>>>>> "Thomas" == Thomas Davie <tom.davie at gmail.com> writes:
>    Thomas> On 12 Mar 2009, at 15:16, Andrew Wagner wrote:
>> Can you expand on this a bit? I'm curious why you think this.
>    Thomas> For two reasons:
>    Thomas> Firstly, I often find that people use the Monadic
>    Thomas> interface when one of the less powerful ones is both
>    Thomas> powerful enough and more convenient, parsec is a wonderful
>    Thomas> example of this.  When the applicative instance is used
>    Thomas> instead of the monadic one, programs rapidly become more
>    Thomas> readable, because they stop describing the order in which
>    Thomas> things should be parsed, and start describing the grammar
>    Thomas> of the language being parsed instead.
> That's interesting.
> I recently used parsec 3. I wrote using the monadic interface because
> I could understand (just about) how to do so. I was looking at the
> examples in RWH, and I could follow the explanation of the monadic
> interface much easier.
> Perhaps this was because RWH shows how to write using the monadic
> interface, and then shows how to convert this to the applicative
> interface.
> It's hard to follow a tutorial that shows you how to convert from
> something you aren't starting with.

I suspect that this is an interesting corner case of both my two  
reasons – firstly, Monads are too powerful here, and secondly, you  
perhaps found it easier to think about the operational aspects of the  
parser than to think about the denotation of what the parser should  

Is that somewhere accurate?


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