I’m glad you are getting back to strictness. Good questions. I’ve pushed (or will as soon as I have validated) a patch that adds type synonyms, updates comments (some of which were indeed misleading), and changes a few names for clarity and consistency. I hope that answers all your questions. Except these: · Why is there a coercion in `HsUnpack` but not in `HsUserBang (Just True) True`? Because the former is implementation generated but the latter is source code specified. · Why isn't this information split over two data types. Because there’s a bit of overlap. See comments with HsSrcBang Simon From: Johan Tibell [mailto:johan.tibell@gmail.com] Sent: 08 January 2015 07:36 To: ghc-devs@haskell.org Cc: Simon Peyton Jones Subject: Clarification of HsBang and isBanged HsBang is defined as: -- HsBang describes what the *programmer* wrote -- This info is retained in the DataCon.dcStrictMarks field data HsBang = HsUserBang -- The user's source-code request (Maybe Bool) -- Just True {-# UNPACK #-} -- Just False {-# NOUNPACK #-} -- Nothing no pragma Bool -- True <=> '!' specified | HsNoBang -- Lazy field -- HsUserBang Nothing False means the same as HsNoBang | HsUnpack -- Definite commitment: this field is strict and unboxed (Maybe Coercion) -- co :: arg-ty ~ product-ty | HsStrict -- Definite commitment: this field is strict but not unboxed This data type is a bit unclear to me: * What are the reasons for the following constructor overlaps? * `HsNoBang` and `HsUserBang Nothing False` * `HsStrict` and `HsUserBang Nothing True` * `HsUnpack mb_co` and `HsUserBang (Just True) True` * Why is there a coercion in `HsUnpack` but not in `HsUserBang (Just True) True`? * Is there a difference in what the user wrote in the case of HsUserBang and HsNoBang/HsUnpack/HsStrict e.g are the latter three generated by the compiler as opposed to being written by the user (the function documentation notwithstanding)? A very related function is isBanged: isBanged :: HsBang -> Bool isBanged HsNoBang = False isBanged (HsUserBang Nothing bang) = bang isBanged _ = True What's the meaning of this function? Is it intended to communicate what the user wrote or whether result of what the user wrote results in a strict function? Context: I'm adding a new StrictData language pragma [1] that makes fields strict by default and a '~' annotation of fields to reverse the default behavior. My intention is to change HsBang like so: - Bool -- True <=> '!' specified + (Maybe Bool) -- True <=> '!' specified, False <=> '~' + -- specified, Nothing <=> unspecified 1. https://ghc.haskell.org/trac/ghc/wiki/StrictPragma -- Johan