CODEWITHSUNDEEP
haskellmonadsstate-monad
hliu
hliu

Reputation: 1047

Stucked in the example of State Monad in Haskell Wiki

I'm learning State Monad and cannot understand one example in Wiki (http://en.wikibooks.org/wiki/Haskell/Understanding_monads/State)

rollDie :: GeneratorState Int
rollDie = do generator <- get
             let (value, newGenerator) = randomR (1,6) generator
             put newGenerator
             return value

the put has definition

put newState = State $ \_ -> ((), newState)

It seems put just create a new State, what the real usage of this line? If want to use the value maybe should use <- to extract, and if want to use the state again should use get. It makes no difference if delete this line (or am i missing anything? ), So, what does this line really mean ?

Upvotes: 2

Views: 161

Answers (1)

Franky
Franky

Reputation: 2376

Delete that put and get some random values. I predict you will always get the same "random value". That is how the random Generator in haskell works: It (i.e. randomR) is a pure function - given the same generator, it will always return the same result. You have to feed the newGenerator to the next call. This is done via put.

Upvotes: 7

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