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

[Brandon S. Allbery KF8NH]

On Nov 15, 2009, at 02:05 , 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?


Because they're not really the same variable; they're separate lambda  
bindings.  If you translate the "do" syntax to the underlying "bind"  
syntax, you get something like:

 > defineVar envRef (id,val) =
 >   liftIO (readIORef envRef) >>=
 >   \env -> return (filter (\(_id,_) -> _id /= id) env) >>=
 >   \env -> liftIO (newIORef val) >>
 >   liftIO (writeIORef envRef ((id,valRef):env)) >>
 >   return val

So you're shadowing (hiding) the original "env" when you reuse the name.

-- 
brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allbery at kf8nh.com
system administrator [openafs,heimdal,too many hats] allbery at ece.cmu.edu
electrical and computer engineering, carnegie mellon university    KF8NH


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