Fix and refactor strict pattern bindings

This patch was triggered by Trac #11601, where I discovered that
-XStrict was really not doing the right thing. In particular,

  f y = let !(Just x) = blah[y] in body[y,x]

This was evaluating 'blah' but not pattern matching it
against Just until x was demanded.  This is wrong.

The patch implements a new semantics which ensures that strict
patterns (i.e. ones with an explicit bang, or with -XStrict)
are evaluated fully when bound.

* There are extensive notes in DsUtils:
  Note [mkSelectorBinds]

* To do this I found I need one-tuples;
  see Note [One-tuples] in TysWiredIn

I updated the user manual to give the new semantics

1 files changed, 5 insertions, 0 deletions

diff --git a/libraries/ghc-prim/GHC/Tuple.hs b/libraries/ghc-prim/GHC/Tuple.hs
index b08d0b4fee..05d6fbfe53 100644
--- a/libraries/ghc-prim/GHC/Tuple.hs
+++ b/libraries/ghc-prim/GHC/Tuple.hs
@@ -26,6 +26,11 @@ default () -- Double and Integer aren't available yet
 -- constructor @()@.
 data () = ()
 
+-- The desugarer uses 1-tuples,
+-- but "()" is already used up for 0-tuples
+-- See Note [One-tuples] in TysWiredIn
+data Unit a = Unit a
+
 data (a,b) = (a,b)
 data (a,b,c) = (a,b,c)
 data (a,b,c,d) = (a,b,c,d)