Clarification of HsBang and isBanged

[Johan Tibell]

Those comments and the renaming really help. Here are a couple of more
questions I got after exploring some more:

DsMeta.repBangTy look wrong to me:

    repBangTy :: LBangType Name -> DsM (Core (TH.StrictTypeQ))
    repBangTy ty= do
      MkC s <- rep2 str []
      MkC t <- repLTy ty'
      rep2 strictTypeName [s, t]
      where
        (str, ty') = case ty of
                       L _ (HsBangTy (HsSrcBang (Just True) True) ty) ->
(unpackedName,  ty)
                       L _ (HsBangTy (HsSrcBang _     True) ty)       ->
(isStrictName,  ty)
                       _                                              ->
(notStrictName, ty)

Shouldn't the second case look at whether -funbox-strict-fields or
-funbox-small-strict-fields is set and use unpackedName instead of
isStrictName if so? What is repBangTy for?

A related question, in MkId.dataConArgRep we have:

    dataConArgRep _ _ arg_ty HsStrict
      = strict_but_not_unpacked arg_ty

Here we're not looking at -funbox-strict-fields
and -funbox-small-strict-fields. Is it the case that we only need to look
at these flags in the case of HsSrcBang, because HsStrict can only be
generated by us (and we presumably looked at the flags when we converted a
HsSrcBang to a HsStrict)?

On Thu, Jan 8, 2015 at 4:09 PM, Simon Peyton Jones <simonpj at microsoft.com>
wrote:

>  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 at gmail.com]
> *Sent:* 08 January 2015 07:36
> *To:* ghc-devs at 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
>
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