[Haskell-cafe] Why can `env` be assigned value two times ?

[zaxis]

defineVar :: Env -> (Id, Val) -> IOThrowsError Val
defineVar envRef (id, val) = do {
    env <- liftIO $ readIORef envRef;
    env <- return $ filter (\(_id, _) -> _id/=id) env; -- clear the current
scope
    valRef <- liftIO $ newIORef val;
    liftIO $ writeIORef envRef $ ((id, valRef):env);
    return val;
}

In haskell, the variable canot change its value , right? If so, why can the
`env` be assigned value twice?

Sincerely!

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fac n = foldr (*) 1 [1..n]
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