recover-rtti: Recover run-time type information from the GHC heap

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The main function in this package is classify, which looks at the GHC heap to recover type information about arbitrary values. This makes it possible for example to show any value (function anythingToString) without having any Show instance in scope, though there are other use cases as well. For example, you could use it to define an anythingToJSON function.

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Debug
- Debug.RecoverRTTI
  - Debug.RecoverRTTI.Classify
  - Debug.RecoverRTTI.ClosureTree

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recover-rtti-0.5.1.tar.gz [browse] (Cabal source package)
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EdskoDeVries, brandonchinn178

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Versions [RSS]	0.1.0.0, 0.2.0.0, 0.2.1.0, 0.3.0.0, 0.4.0.0, 0.4.1.0, 0.4.2, 0.4.3, 0.5.0, 0.5.1, 0.5.2, 0.5.3, 0.6.0 (info)
Change log	CHANGELOG.md
Dependencies	aeson (>=1.4 && <2.3), base (>=4.16 && <4.22), bytestring (>=0.11 && <0.13), containers (>=0.6 && <0.8), ghc-heap (>=9.2 && <9.13), mtl (>=2.2 && <2.4), primitive (>=0.7 && <0.10), sop-core (>=0.5 && <0.6), stm (>=2.5 && <2.6), text (>=1.2 && <2.2), unordered-containers (>=0.2.12 && <0.2.21), vector (>=0.12 && <0.14) [details]
Tested with	ghc ==9.2.8, ghc ==9.4.8, ghc ==9.6.7, ghc ==9.8.4, ghc ==9.10.2, ghc ==9.12.2
License	BSD-3-Clause
Copyright	Juspay Technologies Pvt Ltd, Well-Typed LLP
Author	Edsko de Vries
Maintainer	edsko@well-typed.com
Uploaded	by EdskoDeVries at 2025-07-19T12:07:55Z
Revised	Revision 1 made by brandonchinn178 at 2025-12-14T17:52:45Z
Category	Debugging
Bug tracker	https://github.com/well-typed/recover-rtti/issues
Source repo	head: git clone https://github.com/well-typed/recover-rtti
Distributions	LTSHaskell:0.5.3, NixOS:0.5.3, Stackage:0.6.0
Reverse Dependencies	4 direct, 0 indirect [details]
Downloads	1903 total (31 in the last 30 days)
Rating	(no votes yet) [estimated by Bayesian average]
Your Rating	λ λ λ
Status	Docs available [build log] Last success reported on 2025-07-19 [all 1 reports]

Readme for recover-rtti-0.5.1

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recover-rtti

Recover run-time type information from the GHC heap. The key function in this library is

classify :: a -> Either Closure (Classifier a)

which recovers type information about values about which we know nothing (in particular, no type class constraints). One example use case is the following anythingToString function:

anythingToString :: a -> String

We test that the result of anythingToString is equal to the result of regular show for a large range of data types (including user-defined ones that the library is not aware of). There are also other possible use cases; for example, it should be possible to define an anythingToJSON function.

Obviously there are limitations; the most important ones are:

UNPACKed fields are invisible to the library. This does not need to be a major issue though, when compiling code with -O0 fields are not unpacked. (https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/exts/pragmas.html?highlight=unpack#nounpack-pragma)
Record field names are not known, and so records are shown just using the constructor.

There may be other gotchas as well; this library is primarily intended for debugging.