On 2009 Jan 5, at 15:40, Paul Johnston wrote:
Hi, I was playing around after getting the new O'Reilly book with lists and their operators! Not sure what the below actually means?
Prelude> [] : [] [[]] it :: [[a]]
(Got :set +t on )
I thought the first argument of ':' must be an element, so is the empty list an element of the same type of the contents of the empty list?
(:) :: a -> [a] -> [a] In this case the first argument is an empty list (type forall a. [a]) and the second must therefore be an empty list of lists (type forall a. [[a]]). Hence the result is also of type forall a. [[a]] (with all the `a's unified). -- brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allbery@kf8nh.com system administrator [openafs,heimdal,too many hats] allbery@ece.cmu.edu electrical and computer engineering, carnegie mellon university KF8NH