Document what par undefined x does

[rightfold]

Currently it’s not clear from the documentation what par undefined x does. For example: does it throw an async exception? Does it memoize the exception? Can I rely on Control.Exception.evaluate rethrowing the exception?

[treeowl]

Experimenting a bit in GHCi, it looks like it saves the exception and throws it if and when the undefined value is demanded.

[simonmar]

par undefined x == x