This library provides functions to introspect the Haskell heap, for example to
investigate sharing and lazy evaluation. As this is tied to the internals of
the compiler, it only works with specific versions. Currently, GHC 7.4 through
7.10 should be supported.
It has been inspired by (and taken code from) the vacuum package and the GHCi
debugger, but also allows to investigate thunks and other closures.
This package also provides a new GHCi-command, ':printHeap', which allows you
to inspect the current heap representation of a value, including sharing and
cyclic references. To enable the command, you need to load the included ghci
script or add it to '~/.ghci', as explained by 'cabal install'. Once it is set
up, you can do this:
>> let value = "A Value" >> let x = (value, if head value == 'A' then value
else "", cycle [True, False]) >> :printHeap x >let x1 = _bco > x21 =  >in
(x1,_bco,_bco) >> length (take 100 (show x))
seq return () -- evaluate
everything >> :printHeap x >let x1 = "A Value" > x16 = True : False : x16 >in
You can change the maximum recursion depth using ':setPrintHeapDepth':
>> :setPrintHeapDepth 3 >> :printHeap x >let x1 = C# 'A' : ... : ...
>in (x1,x1,True : ... : ...)
If the view is impaired by blackholes (written '_bh'), running
'System.Mem.performGC' usually helps.
The work on this package has been supported by the Deutsche Telekom Stiftung