Clarification of HsBang and isBanged

[Johan Tibell]

I also note that the definition of isBanged is confusing:

    isBanged :: HsBang -> Bool
    isBanged HsNoBang                  = False
    isBanged (HsUserBang Nothing bang) = bang
    isBanged _                         = True

Why is `HsUserBang (Just False) False`, corresponding to a NOUNPACK
annotations with a missing "!", considered "banged"?

On Thu, Jan 8, 2015 at 8:36 AM, Johan Tibell <johan.tibell at gmail.com> wrote:

> 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
>
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