[Haskell-cafe] return in Monad class necessary?

Chris Eidhof chris at eidhof.nl
Tue Nov 27 04:45:19 EST 2007

On 27 nov 2007, at 10:14, Reinier Lamers wrote:
> Chris Eidhof wrote:
>> On 26 nov 2007, at 19:48, Henning Thielemann wrote:
>>> I wonder whether it is a typical mistake of beginners
>>> to write 'return' within a do-block (that is, not at the end)
>>> and if it is possible to avoid this mistake by clever typing.
>>> In a proper monad 'return' can be fused with subsequent actions,
>>> and thus it is not necessary within a sequence of actions.
>>> However, although sensible, 'return' is also not required at the  
>>> end  of a block.
>>> Has someone already thought about a replacement for monads?
>> I also made that mistake in the beginning, I used return instead  
>> of  lets. I don't think it's a big problem, most users will find  
>> out once  they've got some more experience, and it doesn't really  
>> do any harm.
> It might be possible for the compiler to emit a warning when a  
> return is used in the middle of a do block as the top level operator  
> on a line. OTOH, that still wouldn't catch something like "when (x  
> == 0) (return ())" which doesn't do what an imperative programmer  
> expects.
Well, there are two things about the return:

First, some people want to use return just as an imperative programmer  
would use it: to exit from a function. So the programmer doesn't  
expect the commands after that return are executed.

Second, the problem I had was that I didn't know how to do  
computations with the data I got from the monad, for example:

 > main = do
 >   myLine <- getLine
 >   reversed <- return $ unwords $ reverse $ words myLine
 >   putStrLn reversed

Instead of the 3rd line I could have written
 > let reversed = unwords $ reverse $ words myLine

This is another problem, but it doesn't affect the computation,  
whereas the first problem is more serious.


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