Hi, At least with the current situation, it is possible (from my experience so far, but I don't claim to have tested extensively) to write code that works both with the old class hierarchy as well as the new: * Add an import Control.Applicative * Add an explicit method definition "return = pure" for Monad instances * Add Applicative to contexts in a few strategic places. That's a big plus in my books for people who for one reason or another would like to write code that works against an as broad range of different versions of the available Haskell tools as possible. On the other hand, I don't really understand the rationale:
Traditionally, `return` is often used where `pure` would suffice today, forcing a `Monad` constraint even if a weaker `Applicative` would have sufficed.
As a result, language extensions like `ApplicativeDo`[3] have to rewrite `return` to weaken its `Monad m =>` constraint to `Applicative m =>` in order to benefit existing code at the cost of introducing magic behavior at the type level.
If code is genuinely applicative, why not encourage this to be clearly signalled by use of "pure" instead of "return"? Surely that would benefit long-term readability a lot more than supporting use of the "wrong" function. And as ApplicativeDo is a new extension, removing the magic so as to force such a change for code that really is applicative ought not to cause that much trouble for existing code. Thus, encourage (or force) people to say what they mean by using "pure" and "return" appropriately. I'm thus (very) unconvinced about the merits of this proposal. Best, /Henrik -- Henrik Nilsson School of Computer Science The University of Nottingham nhn@cs.nott.ac.uk This message and any attachment are intended solely for the addressee and may contain confidential information. If you have received this message in error, please send it back to me, and immediately delete it. Please do not use, copy or disclose the information contained in this message or in any attachment. Any views or opinions expressed by the author of this email do not necessarily reflect the views of the University of Nottingham. This message has been checked for viruses but the contents of an attachment may still contain software viruses which could damage your computer system, you are advised to perform your own checks. Email communications with the University of Nottingham may be monitored as permitted by UK legislation.