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Diffstat (limited to 'compiler/GHC/Types/Unique.hs')
| -rw-r--r-- | compiler/GHC/Types/Unique.hs | 16 |
1 files changed, 14 insertions, 2 deletions
diff --git a/compiler/GHC/Types/Unique.hs b/compiler/GHC/Types/Unique.hs index cc013dc9a1..4d19ba7f66 100644 --- a/compiler/GHC/Types/Unique.hs +++ b/compiler/GHC/Types/Unique.hs @@ -68,8 +68,20 @@ import Data.Bits * * ************************************************************************ -The @Chars@ are ``tag letters'' that identify the @UniqueSupply@. -Fast comparison is everything on @Uniques@: +Note [Uniques and masks] +~~~~~~~~~~~~~~~~~~~~~~~~ +A `Unique` in GHC is a Word-sized value composed of two pieces: +* A "mask", of width `UNIQUE_TAG_BITS`, in the high order bits +* A number, of width `uNIQUE_BITS`, which fills up the remainder of the Word + +The mask is typically an ASCII character. It is typically used to make it easier +to distinguish uniques constructed by different parts of the compiler. +There is a (potentially incomplete) list of unique masks used given in +GHC.Builtin.Uniques. See Note [Uniques-prelude - Uniques for wired-in Prelude things] + +`mkUnique` constructs a `Unique` from its pieces + mkUnique :: Char -> Int -> Unique + -} -- | Unique identifier. |