multithreading - Sharing a mvar between threads -

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i'm trying make program print arrows until user press enter (see code bellow).

the problem when press enter, see "stop" string in console, doesn't change value of m in outputarrows function.

how can share state?

import control.concurrent import control.concurrent.async import control.monad  waitforinput m =     getline     putstrln "stop"     putmvar m true  outputarrows m =     stop <- readmvar m     unless stop $         threaddelay 1000000         putstr ">"         outputarrows m  main =     m <- newmvar false     th1 <- async (waitforinput m)     th2 <- async (outputarrows m)     wait th1     wait th2

your putmvar doesn't put new value in mvar blocks indefinitely. mvars boxes can hold single value. if want replace value, need take out old value first.

if don't need blocking behavior of mvar, should use regular ioref or possibly tvar if need ensure more complex operations run atomically.


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