indexArray# getting poorly deferred

In both of the following functions I am attempting to execute a read from an Array# a and then write the result to a MutableArray# s a, and I would like the following evaluation properties:

• the a value is not forced, i.e. the value stored in the MutableArray# s a will remain a thunk if the original value in the Array# a was; and
• the new MutableArray# does not maintain any references to the original Array#, so that the original can be GCed.

The singleton-unboxed-tuple return type of indexArray# appears to be there to allow specifically this use case, but sadly it only succeeds for the first of these functions. The second function ends up storing a thunk in the MutableArray# which contains a reference to the Array#.

Haskell:

{-# LANGUAGE MagicHash #-}
{-# LANGUAGE UnboxedTuples #-}

module IndexThenWrite where

import GHC.Prim

indexThenWrite :: Array# a -> MutableArray# s a -> State# s -> State# s
indexThenWrite arr marr s = case indexArray# arr 123# of
  (# a #) -> writeArray# marr 234# a s

indexThenWriteF :: (a -> b) -> Array# a -> MutableArray# s b -> State# s -> State# s
indexThenWriteF f arr marr s = case indexArray# arr 123# of
  (# a #) -> writeArray# marr 234# (f a) s

Core:

-- RHS size: {terms: 15, types: 18, coercions: 0, joins: 0/0}
indexThenWrite
indexThenWrite
  = \ @ a_aq9 @ s_aqa arr_apa marr_apb s1_apc ->
      case indexArray# arr_apa 123# of { (# ipv_sUH #) ->
      writeArray# marr_apb 234# ipv_sUH s1_apc
      }

-- RHS size: {terms: 18, types: 22, coercions: 0, joins: 0/0}
indexThenWriteF
indexThenWriteF
  = \ @ a_aq0 @ b_aq1 @ s_aq2 f_ape arr_apf marr_apg s1_aph ->
      writeArray#
        marr_apg
        234#
        (case indexArray# arr_apf 123# of { (# ipv_sUK #) ->
         f_ape ipv_sUK
         })
        s1_aph

I'd like the second function to generate code similar to the first.

Trac metadata

Trac field	Value
Version	8.2.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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