GitLab

Currently, the template-haskell API is lagging behind what is possible with GHC's strictness annotations in data types, especially since the advent of StrictData. Currently, template-haskell has Strict:

data Strict
  = IsStrict	 
  | NotStrict	 
  | Unpacked

But it appears that there are actually nine different combinations of packedness and strictness annotations:

data A = A                   Int -- No unpackedness, no strictness
data A = A                  !Int -- No unpackedness, strict
data A = A                  ~Int -- No unpackedness, lazy
data A = A {-# NOUNPACK #-}  Int -- NOUNPACK, no strictness
data A = A {-# NOUNPACK #-} !Int -- NOUNPACK, strict
data A = A {-# NOUNPACK #-} ~Int -- NOUNPACK, lazy
data A = A {-#   UNPACK #-}  Int -- UNPACK, no strictness
data A = A {-#   UNPACK #-} !Int -- UNPACK, strict
data A = A {-#   UNPACK #-} ~Int -- UNPACK, lazy

It seems like the most consistent thing to do would be change Strict and add Unpack to the template-haskell API:

data Strict
  = IsStrict
  | IsLazy
  | NoStrictAnnot

data Unpack
  = Unpack
  | NoUnpack
  | NoUnpackAnnot

type UnpackStrict        = (Unpack, Strict)
type UnpackStrictType    = (UnpackStrict, Type)
type VarUnpackStrictType = (Name, UnpackStrict, Type)

type StrictType    = UnpackStrictType
type VarStrictType = VarUnpackStrictType

And so on.