Brian Bloniarz wrote:
I got confused by the GHC documentation recently, I was wondering how it could be improved. From: http://www.haskell.org/ghc/docs/latest/html/users_guide/bang-patterns.html
<cite> The let-binding can be recursive. However, it is much more common for the let-binding to be non-recursive, in which case the following law holds: (let !p = rhs in body) is equivalent to (case rhs of !p -> body) </cite> Shouldn't the bang be removed in the final case pattern? Furthermore with existential types the let binding is not supported: data E = forall a . Show a => E a f :: E -> String f x = case x of E a -> show a f works, but g g :: E -> String g x = let !(E a) = x in show a fails (with or without the bang): My brain just exploded. I can't handle pattern bindings for existentially-quantified constructors. Instead, use a case-expression, or do-notation, to unpack the constructor. In the binding group for !(E a) In a pattern binding: !(E a) = x In the expression: let !(E a) = x in show a In the definition of `g': g x = let !(E a) = x in show a Cheers Christian P.S. It should be mentioned that ~ and ! only make sense for single variant data types (like tuples)