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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