Strange behavior of GC under ghci

Releasing the result of newForeignPtr nullFunPtr nullPtr end in core dump, due to the fact that finalizer function is set to null pointer.

When I run something like this in GHCI:

> import System.Mem
> import Foreign.Ptr 
> import Foreign.ForeignPtr
> import System.IO.Unsafe
> import qualified Data.Map as Map
> a <- return $ Map.singleton 1 (unsafePerformIO $ newForeignPtr nullFunPtr nullPtr)
Loading package array-0.5.0.0 ... linking ... done.
Loading package deepseq-1.3.0.2 ... linking ... done.
Loading package containers-0.5.5.1 ... linking ... done.
> print a
fromList [(1,0x0000000000000000)]
> performGC
> ^D
Leaving GHCi.
[1]    3782 segmentation fault (core dumped)  ghci

it wont crash until exit from ghci which is correct. But if I do something similar but using let binding it will crash even if variable a didn't lose its scope

> import System.Mem
> import Foreign.Ptr 
> import Foreign.ForeignPtr
> import System.IO.Unsafe
> import qualified Data.Map as Map
> let a = Map.singleton 1 (unsafePerformIO $ newForeignPtr nullFunPtr nullPtr)
Loading package array-0.5.0.0 ... linking ... done.
Loading package deepseq-1.3.0.2 ... linking ... done.
Loading package containers-0.5.5.1 ... linking ... done.
> print a
fromList [(1,0x0000000000000000)]
[1]    3842 segmentation fault (core dumped)  ghci

Why is there a difference between doing it with do notation and with let binding?

I also expected that if I rebind variable a it will lose it's scope and will be released but it is not (see below)

> import System.Mem
> import Foreign.Ptr 
> import Foreign.ForeignPtr
> import System.IO.Unsafe
> import qualified Data.Map as Map
> a <- return $ Map.singleton 1 (unsafePerformIO $ newForeignPtr nullFunPtr nullPtr)
Loading package array-0.5.0.0 ... linking ... done.
Loading package deepseq-1.3.0.2 ... linking ... done.
Loading package containers-0.5.5.1 ... linking ... done.
> print a
fromList [(1,0x0000000000000000)]
> a <- return () -- rebinding varable a, it is no longer used
> performGC
>  -- no crash, varaible a not released

Trac metadata

Trac field	Value
Version	7.8.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	GHCi
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	hvr
Operating system
Architecture