Yeah it is possible to do it with another domonad block. I just don't want
to clutter up the code by repeating myself.
I made a macro do-state that introduces a new domonad block using the
current state. It works:
(defmacro do-state [bindings & r]
`(fn [st] ( (domonad state-m ~bindings ~@r) st)))
(domonad state-m
[a (fetch-val :a)
_ (if a
(do-state[ _ (update-state #(assoc % :b 2))
_ (update-state
#(assoc % :b 2))]
nil)
(m-result nil))
]
nil)
On Thursday, July 11, 2013 2:18:44 PM UTC-7, Ben wrote:
>
> Is it not possible just to put another domonad block in the true branch of
> the if? Alternately, it should be possible to do it with an explicit bind
> operation.
>
>
> On Thu, Jul 11, 2013 at 2:11 PM, JvJ <[email protected] <javascript:>>wrote:
>
>> I'm trying to use a domonad block with the state monad, and I'm running
>> into trouble chaining together multiple monadic instructions.
>>
>> Take this for example:
>>
>> ;; This block of code fetches the value from :a in the state,
>> ;; and then associates a new value for :b if :a is set.
>> (domonad state-m
>> [a (fetch-val :a)
>> _ (if a
>> (update-state #(assoc % :b 2))
>> (m-result nil))
>> ]
>> nil)
>>
>> My question is, what if I wanted to run two update functions, like
>> (update-state #(assoc % :b 2)) and (update-state #(assoc % :c 3)),
>> but within the if statement?
>>
>> (p.s. I realize that assoc is variadic, and could easily deal with this,
>> however I'm using
>> it as a simple example of a more general case)
>>
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>
>
>
> --
> Ben Wolfson
> "Human kind has used its intelligence to vary the flavour of drinks, which
> may be sweet, aromatic, fermented or spirit-based. ... Family and social
> life also offer numerous other occasions to consume drinks for pleasure."
> [Larousse, "Drink" entry]
>
>
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