Hmmm. So you think maybe it's doing something like (roughly; I don't have the exact case in front of me) (\p q -> f p (g p q)) this that ==> beta reduce let p = this q = thatin f p (g p q) ==> inline q let p = thisin f p (g p that)? I tried marking the "this" in question INLINE CONLIKE [0]. Shouldn't that tell GHC that duplicating it is fine? I really want to see it for RULES. David FeuerWell-Typed, LLP -------- Original message --------From: Simon Peyton Jones <simonpj@microsoft.com> Date: 3/26/18 7:14 PM (GMT-05:00) To: David Feuer <david@well-typed.com>, ghc-devs@haskell.org Subject: RE: Missed beta reductions GHC always beta-reduces. It does not always inline. For that: -ddump-inlinings Simon From: ghc-devs <ghc-devs-bounces@haskell.org> On Behalf Of David Feuer Sent: 26 March 2018 23:59 To: ghc-devs@haskell.org Subject: Missed beta reductions What's the best way to find spots where GHC saw a redex and choose not to beta reduce? Is there a flag for that? It could be useful when trying to figure out why rules aren't firing. David Feuer Well-Typed, LLP