[Haskell-cafe] ANN: The Typeclassopedia, and request for feedback

Adam Vogt vogt.adam at gmail.com
Tue Feb 17 21:30:32 EST 2009

* On Monday, February 16 2009, Andrew Coppin wrote:

> I do have one little question. Let me see if I can find the quote... Ah,  
> here we go:
>  "The WrappedMonad and WrappedArrow constructors witness the fact that  
> any Monad and any Arrow can be made into an Applicative."
> I don't really understand that sentence. Care to elaborate?

imho, the original wording is concise and clear, but with more common 

Arrow and Monad are special cases of Functor and Applicative, so you can 
make an Applicative out of any Arrow or Monad. Those newtypes have 
Applicative instances which you can use for an arbitrary Monad or Arrow, 
when they are inside that newtype.

You could also clarify which part you don't understand (the meanings of 
constructor, witness, the classes involved, etc.).

If the linked haddock documentation had links to colorized source code 
(present in previous versions), then looking up these instances might be 

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