Generally speaking, all Haskell functions are pure unless they use unsafe- functions or FFI inside. Отправлено с iPhone Nov 24, 2010, в 23:46, Gregory Propf написал(а):

I have a pretty basic question. I've been wondering about whether monadic functions that do NOT us IO can be pure or not. There seems to be some confusion on this topic on the web. I'm especially interested in whether they can be memoized. It seems to me that something like a function in the State monad should be pure provided the same initial state and same function arguments are present. Likewise with the list monad and most other monads in fact.

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On Wed, Nov 24, 2010 at 12:46:22PM -0800, Gregory Propf wrote:

I have a pretty basic question.  I've been wondering about whether monadic functions that do NOT us IO can be pure or not.  There seems to be some confusion on this topic on the web.  I'm especially interested in whether they can be memoized.  It seems to me that something like a function in the State monad should be pure provided the same initial state and same function arguments are present.  Likewise with the list monad and most other monads in fact.

From the point of view of *using* a monad, code written using some monads may *look like* it is impure. For example, the State monad's 'put' sure looks a lot like mutating a reference cell. And that's the

The confusion stems from the fact that there are two points of view from which to look at any monad. From the point of view of *implementation*, all Haskell monads other than IO are completely pure: after all, they are just plain old Haskell code. For example, the State monad is implemented as a function that takes an old state and returns a computed value and a new state. No tricks, no mutation, just plain functions and parameter passing. point; the State monad lets us think and program as if we are really mutating a reference cell. But of course under the hood nothing of the sort is really happening. It's the same with every monad: each one models some sort of "effect" in a pure way. So, the purity of code using monads depends on your point of view. If you're worried about whether your code is going to send messages over the network or give different answers when called multiple times with the same inputs, the answer is no. If you're worried about composability and reasoning about your programs, then you should worry about extensive use of (say) the state monad for the same reason that you should avoid global variables in an imperative language. -Brent