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<p>It seems like you need your write-state to be a monoid in order
to infer a `MonadRandom` instance. Based on your code, I'm
guessing the compiler can't infer that.<br>
<br>
Have a look at
<a class="moz-txt-link-freetext" href="https://hackage.haskell.org/package/MonadRandom-0.5.1.1/docs/Control-Monad-Random-Class.html#t:MonadRandom">https://hackage.haskell.org/package/MonadRandom-0.5.1.1/docs/Control-Monad-Random-Class.html#t:MonadRandom</a><br>
</p>
<br>
<div class="moz-cite-prefix">On 08/03/2018 07:31 AM, Sergiu Ivanov
wrote:<br>
</div>
<blockquote type="cite" cite="mid:87600rskm7.fsf@colimite.fr">
<pre wrap="">Dear Café,
I would like to use random-fu to do some pseudo-random simulations for a
given StdGen (so that I can run the same simulation multiple times, if
needed).
The following works:
testState :: StdGen -> Int
testState = evalState (sampleRVar $ uniform 1 10)
The following doesn't:
testRWS :: StdGen -> Int
testRWS = fst . evalRWS (sampleRVar $ uniform 1 10) ()
I get
<interactive>:2:26: error:
• No instance for (MonadRandom (RWST () b0 StdGen Identity)) arising from a use of ‘sampleRVar’
• In the first argument of ‘evalRWS’, namely ‘(sampleRVar $ uniform 1 10)’
In the second argument of ‘(.)’, namely ‘evalRWS (sampleRVar $ uniform 1 10) ()’
In the expression: fst . evalRWS (sampleRVar $ uniform 1 10) ()
Indeed, I do see a MonadRandom instance for StateT, but none for RWST [0].
Is there a reason to not have a MonadRandom instance for RWST?
Am I looking in the wrong place?
-
Sergiu
[0] <a class="moz-txt-link-freetext" href="https://hackage.haskell.org/package/random-source-0.3.0.6/docs/Data-Random-Source.html">https://hackage.haskell.org/package/random-source-0.3.0.6/docs/Data-Random-Source.html</a>
</pre>
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