+'mkCoreAppDs' and 'mkCoreAppsDs' hand the special-case desugaring of 'seq'.
+
+Note [Desugaring seq (1)] cf Trac #1031
+~~~~~~~~~~~~~~~~~~~~~~~~~
+ f x y = x `seq` (y `seq` (# x,y #))
+
+The [CoreSyn let/app invariant] means that, other things being equal, because
+the argument to the outer 'seq' has an unlifted type, we'll use call-by-value thus:
+
+ f x y = case (y `seq` (# x,y #)) of v -> x `seq` v
+
+But that is bad for two reasons:
+ (a) we now evaluate y before x, and
+ (b) we can't bind v to an unboxed pair
+
+Seq is very, very special! So we recognise it right here, and desugar to
+ case x of _ -> case y of _ -> (# x,y #)
+
+Note [Desugaring seq (2)] cf Trac #2231
+~~~~~~~~~~~~~~~~~~~~~~~~~
+Consider
+ let chp = case b of { True -> fst x; False -> 0 }
+ in chp `seq` ...chp...
+Here the seq is designed to plug the space leak of retaining (snd x)
+for too long.
+
+If we rely on the ordinary inlining of seq, we'll get
+ let chp = case b of { True -> fst x; False -> 0 }
+ case chp of _ { I# -> ...chp... }
+
+But since chp is cheap, and the case is an alluring contet, we'll
+inline chp into the case scrutinee. Now there is only one use of chp,
+so we'll inline a second copy. Alas, we've now ruined the purpose of
+the seq, by re-introducing the space leak:
+ case (case b of {True -> fst x; False -> 0}) of
+ I# _ -> ...case b of {True -> fst x; False -> 0}...
+
+We can try to avoid doing this by ensuring that the binder-swap in the
+case happens, so we get his at an early stage:
+ case chp of chp2 { I# -> ...chp2... }
+But this is fragile. The real culprit is the source program. Perhaps we
+should have said explicitly
+ let !chp2 = chp in ...chp2...
+
+But that's painful. So the code here does a little hack to make seq
+more robust: a saturated application of 'seq' is turned *directly* into
+the case expression. So we desugar to:
+ let chp = case b of { True -> fst x; False -> 0 }
+ case chp of chp { I# -> ...chp... }
+Notice the shadowing of the case binder! And now all is well.
+
+The reason it's a hack is because if you define mySeq=seq, the hack
+won't work on mySeq.
+
+Note [Desugaring seq (3)] cf Trac #2409
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+The isLocalId ensures that we don't turn
+ True `seq` e
+into
+ case True of True { ... }
+which stupidly tries to bind the datacon 'True'.
+
+\begin{code}
+mkCoreAppDs :: CoreExpr -> CoreExpr -> CoreExpr
+mkCoreAppDs (Var f `App` Type ty1 `App` Type ty2 `App` arg1) arg2
+ | f `hasKey` seqIdKey -- Note [Desugaring seq (1), (2)]
+ = Case arg1 case_bndr ty2 [(DEFAULT,[],arg2)]
+ where
+ case_bndr = case arg1 of
+ Var v1 | isLocalId v1 -> v1 -- Note [Desugaring seq (2) and (3)]
+ _ -> mkWildBinder ty1
+
+mkCoreAppDs fun arg = mkCoreApp fun arg -- The rest is done in MkCore
+
+mkCoreAppsDs :: CoreExpr -> [CoreExpr] -> CoreExpr
+mkCoreAppsDs fun args = foldl mkCoreAppDs fun args
+\end{code}
+
+
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