[Haskell-cafe] indexed writer monad

Wed Mar 25 16:03:11 UTC 2015

Sorry, I didn't mean to scare you off.

By Category, I didn't mean the math concept; I meant simply the class
from the Control.Category module. You don't need to learn any category
theory to understand it. Since you seem to know already what Monoid is,
try to compare those two classes, notice the similarities and see how
(and why) one could be called an indexed version of the other.

But if you don't know what "indexed" means, how do you know you need it?
Perhaps you could describe your problem, and we could tell you what
abstraction would fit that use case, be it indexed monad or something else.

On 25/03/15 17:22, Nicholls, Mark wrote:
> Ah that assumes I know what a category is! (I did find some code that claimed the same thing)....maths doesn't scare me (much), and I suspect its nothing complicated (sounds like a category is a tuple then! Probably not), but I don't want to read a book on category theory to write a bit of code...yet.
> 
> Ideally there would be a chapter in something like "learn you an indexed Haskell for the great good".
> 
> Then I could take the code, use it...mess about with it...break it...put it back together in a slightly different shape and....bingo...it either works...or I find theres a good reason why it doesn't....(or I post a message to the café). 
> 
> -----Original Message-----
> From: Roman Cheplyaka [mailto:roma at ro-che.info] 
> Sent: 25 March 2015 2:46 PM
> To: Nicholls, Mark; haskell-cafe at haskell.org
> Subject: Re: [Haskell-cafe] indexed writer monad
> 
> An indexed monoid is just a Category.
> 
> On 25/03/15 16:32, Nicholls, Mark wrote:
>> Anyone?
>>
>>  
>>
>> I can handle monads, but I have something (actually in F#) that feels 
>> like it should be something like a indexed writer monad (which F# 
>> probably wouldn't support).
>>
>>  
>>
>> So I thought I'd do some research in Haskell.
>>
>>  
>>
>> I know little or nothing about indexed monad (though I have built the 
>> indexed state monad in C#).
>>
>>  
>>
>> So I would assume there would be an indexed monoid (that looks at bit 
>> like a tuple?).
>>
>>  
>>
>> e.g.
>>
>>  
>>
>> (a,b) ++ (c,d) = (a,b,c,d)
>>
>> (a,b,c,d) ++ (e) = (a,b,c,d,e)
>>
>>  
>>
>> ?
>>
>>  
>>
>> There seems to be some stuff about "update monads", but it doesn't 
>> really look like a writer.
>>
>>  
>>
>> I could do with playing around with an indexed writer, in order to get 
>> my head around what I'm doing..then try and capture what I'm 
>> doing.then try (and fail) to port it back.
>>
>>  
>>
>>  
>>
>>
>>
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