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

[zaxis]

thanks for all your answers !


zaxis wrote:
> 
> 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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