--- The Int is number of value args the thing can be
--- applied to without doing much work
---
--- This is used when eta expanding
--- e ==> \xy -> e x y
---
--- It returns 1 (or more) to:
--- case x of p -> \s -> ...
--- because for I/O ish things we really want to get that \s to the top.
--- We are prepared to evaluate x each time round the loop in order to get that
-
--- It's all a bit more subtle than it looks. Consider one-shot lambdas
--- let x = expensive in \y z -> E
--- We want this to have arity 2 if the \y-abstraction is a 1-shot lambda
--- Hence the ArityType returned by arityType
-
--- NB: this is particularly important/useful for IO state
--- transformers, where we often get
--- let x = E in \ s -> ...
--- and the \s is a real-world state token abstraction. Such
--- abstractions are almost invariably 1-shot, so we want to
--- pull the \s out, past the let x=E.
--- The hack is in Id.isOneShotLambda
---
--- Consider also
--- f = \x -> error "foo"
--- Here, arity 1 is fine. But if it is
--- f = \x -> case e of
--- True -> error "foo"
--- False -> \y -> x+y
--- then we want to get arity 2.
--- Hence the ABot/ATop in ArityType
+{- The Arity returned is the number of value args the
+ thing can be applied to without doing much work
+
+exprEtaExpandArity is used when eta expanding
+ e ==> \xy -> e x y
+
+It returns 1 (or more) to:
+ case x of p -> \s -> ...
+because for I/O ish things we really want to get that \s to the top.
+We are prepared to evaluate x each time round the loop in order to get that
+
+It's all a bit more subtle than it looks:
+
+1. One-shot lambdas
+
+Consider one-shot lambdas
+ let x = expensive in \y z -> E
+We want this to have arity 2 if the \y-abstraction is a 1-shot lambda
+Hence the ArityType returned by arityType
+
+2. The state-transformer hack
+
+The one-shot lambda special cause is particularly important/useful for
+IO state transformers, where we often get
+ let x = E in \ s -> ...
+
+and the \s is a real-world state token abstraction. Such abstractions
+are almost invariably 1-shot, so we want to pull the \s out, past the
+let x=E, even if E is expensive. So we treat state-token lambdas as
+one-shot even if they aren't really. The hack is in Id.isOneShotLambda.
+
+3. Dealing with bottom
+
+Consider also
+ f = \x -> error "foo"
+Here, arity 1 is fine. But if it is
+ f = \x -> case x of
+ True -> error "foo"
+ False -> \y -> x+y
+then we want to get arity 2. Tecnically, this isn't quite right, because
+ (f True) `seq` 1
+should diverge, but it'll converge if we eta-expand f. Nevertheless, we
+do so; it improves some programs significantly, and increasing convergence
+isn't a bad thing. Hence the ABot/ATop in ArityType.
+
+Actually, the situation is worse. Consider
+ f = \x -> case x of
+ True -> \y -> x+y
+ False -> \y -> x-y
+Can we eta-expand here? At first the answer looks like "yes of course", but
+consider
+ (f bot) `seq` 1
+This should diverge! But if we eta-expand, it won't. Again, we ignore this
+"problem", because being scrupulous would lose an important transformation for
+many programs.
+-}