Version 2 (modified by jstolarek, 5 years ago) (diff)


Implementing primitive Bool#

This page gathers the notes about implementing Bool as a primitive data type and thus resolving ticket #6135. The motivation is to have primitive logical operators AND, OR and NOT that are strict in all arguments and compute their result by means of primitive bitwise operations. This would eliminate need for branching present in current implementation of logical operators:

(||) x y
 = case x of
        True  -> True
        False -> y

This would prevent code duplication caused by case-of-case transformation when multiple logical operations are chained together (see discussion on ticket #6135 for examples).

Possible solutions and their consequences

Below are possible approaches to implement primitive Bool# in Haskell source language:

First approach

Treat Bool as a boxed version of primitive Bool#. True would be equivalent of B# True#, False of B# False#:

data Bool = B# True# | B# False# 

-- B# :: Bool# -> Bool

Not sure if this can be considered equivalent to what the Haskell Report says about Bool. We need to ensure that Bool# is populated only by True# and False# and that these two are translated to 1# and 0# in the Core. It should be impossible to write such a function at Haskell level:

g :: Bool -> Int -> Int
g (B# b) (I# i) = I# (b + i)

This approach might require one additional case expression to inspect the value of Bool at the Core level. For example:

f :: Int -> Int -> Int
f x y = if x > y
        then x
        else y

would compile to:

case x of _ { I# xP ->
 case y of _ { I# yP ->
   case ># xP yP of _ {
     B# bP -> case bP of _ { 1# -> e1; 0# -> e2 }

This would complicate Core a bit but it should be possible to compile such Core to exactly the same result as with normal Bool. This code assumes that ># has type Int# -> Int# -> Bool, but to truly avoid branching in the Core we need .># :: Int# -> Int# -> Bool# so that we get a primitive value that doesn't need to be inspected using case expression but can be directly used by primitive logical operators.

Second approach

Second approach assumes creating type Bool# that is independent of type Bool. Boxing and unboxing would have to be done explicitly via additional functions:

data Bool = True | False -- no changes here

bBox :: Bool# -> Bool
bBox 1# = True
bBox 0# = False

bUnbox :: Bool -> Bool#
bUnbox True  = 1#
bUnbox False = 0#

Bool# could not be implemented as an ADT because it is unlifted and unboxed, while ADT value constructors need to be boxed and lifted (see comments in compiler/types/TyCon.lhs). There would need to be some magical way of ensuring that Bool# is populated only by #0 and 1# and that these values cannot be mixed with unboxed integers. Perhaps this could be done by preventing programmer from explicitly creating values of that type (can this be done?) and allow her only to use values returned from functions.

Another problem with this approach is that it would introduce primitive logical operations
# and &&# with type Int# -> Int# -> Int# - it is questionable whether anyone would want such operations available to the programmer. I think it is desirable to have primitive logical operators of type Bool# -> Bool# -> Bool#.

Places of interest in the source code

The file prelude/primops.txt.pp defines PrimOps and their type signatures. An example definition looks like this:

primop   IntGtOp  ">#"   Compare   Int# -> Int# -> Bool
   with fixity = infix 4

Existing definitions should remain unchanged or the code using them would break and that is a Very Bad Thing. This would require creating new PrimOps:

primop   IntGtOpB  ".>#"   Compare   Int# -> Int# -> Bool#
   with fixity = infix 4

The tricky part here is Compare. This a value constructor of PrimOpInfo data type defined in prelude/PrimOp.lhs:

data PrimOpInfo
  = Dyadic      OccName         -- string :: T -> T -> T
  | Monadic     OccName         -- string :: T -> T
  | Compare     OccName         -- string :: T -> T -> Bool
  | GenPrimOp   OccName         -- string :: \/ . T1 -> .. -> Tk -> T

We would need new PrimOpInfo value to denote PrimOps of type T -> T -> Bool#. Appropriate functions like primOpSig and getPrimOpResultInfo would have to be adjusted accordingly.

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