recover-rtti

Recover run-time type information from the GHC heap

LTS Haskell 24.33:0.5.3
Stackage Nightly 2026-03-05:0.6.0
Latest on Hackage:0.6.0

See all snapshots recover-rtti appears in

BSD-3-Clause licensed by Edsko de Vries
Maintained by [email protected]
This version can be pinned in stack with:recover-rtti-0.6.0@sha256:c147547e9a867d3adb9d7e155d6b8cc09104ac3085b4be91e4eafc744411f933,4294

Module documentation for 0.6.0

Used by 1 package in nightly-2026-03-05(full list with versions):
skeletest

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:

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

Changes

Revision history for recover-rtti

0.6.0 – 2026-03-05

THIS IS AN IMPORTANT BUGFIX RELEASE; PLEASE UPGRADE. Previous versions of recover-rtti may result in segfaults. Details below.

This release changes how we classify containers. In versions prior to 0.6, we would classify a list such as [True, False] as C_List (C_Prim C_Bool). We did this by looking at the first element of the list, if one existed; if the list was empty, we’d classify it as C_List C_Void. This is however not always correct. Suppose we have a list of lists, and the first inner list happens to be empty. We’d then classify the list of lists as C_List (C_List C_Void), but that is of course wrong: the next inner list might not be empty, and classifying it as [Void] (and then attempting to print it) could result in segfaults.

Starting in version 0.6 we defer classification of type arguments, merely classifying a list as C_List, implying that its type is [Deferred]. Specific applications, such as anythingToString, will then recursively classify each element prior to printing it.

We do make two exceptions to this rule:

  • For lists and list-like structures, we do check if the elements are (all) of type Char, so that the overlapping instance Show for [String] (versus [a]) can be used. Not doing this would result in significantly less useful output from anythingToString.
  • To distinguish HashMap from HashSet we look at the first element only. At least for printing this cannot result in segfaults (it would just mean that the values of the HashMap are omitted), and it’s anyway exceedingly unlikely to happen in the first place.

Since each element is individually classified, there is no need for the BoxAnything workaround anymore, which has therefore been deleted.

With thanks to Brandon Chinn for the report and the minimal reproducer (#51).

0.5.3 – 2026-01-07

  • Support ghc-9.14 (Brandon Chinn, #48)
  • Support containers-0.8 (Brandon Chinn, #49)

0.5.2 – 2025-12-16

  • Support unordered-containers-0.2.21 (Brandon Chinn, #45)

0.5.1 – 2025-07-19

  • Relax bounds, including support for QuickCheck-2.16
  • Drop support for ghc < 9.2

0.5.0 – 2024-06-12

  • Support ghc 9.8 and 9.10, including bytestring 0.12
  • Support ByteArray and MutableByteArray
  • Relax bounds (Marcin Szamotulski, #38)
  • Drop support for ghc 8.8

0.4.3 – 2023-06-05

  • Support aeson 2.1, vector 0.13, and primitive 0.8 (Mitchell Rosen, #29, #35, #36)
  • Support bytestring 0.11.4
  • Support ghc 9.4 and 9.6

0.4.2 – 2023-03-23

  • Support mtl 2.3 (requiring at least 2.3.1) (#26). Thanks to Rebecca Turner.

0.4.1 – 2022-03-17

  • Support for ghc 9.2, bytestring 0.11, aeson 0.2
  • Added traceAnything and traceAnythingId (#13)
  • Added deriving-via support (AnythingToString, BoxAnything) (#3)

0.4 – 2021-06-30

  • Correctly set some required lower bounds.
  • Add support for reclassification
  • Add classification equality check
  • Add support for primitive arrays and vectors
  • Fix classification on OSX
  • General internal cleanup of the library

This release is backwards incompatible with 0.3, but users that simply use anythingToString should be unaffected.

0.3.0.0 – 2021-03-17

  • Fix bug that could cause anythingToString to fail on lists with an elemUserDefined exception (issue #6). Thanks to Bit Connor for the report!

0.2.1.0 – 2021-03-17

  • Add support for unordered-containers and boxed vectors.

0.2.0.0 – 2021-03-15

  • Reorganize module structure (primarily to improve Haddocks)

0.1.0.0 – 2021-03-11

  • Alpha release