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