Clarification of HsBang and isBanged

[Simon Peyton Jones]

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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