HsYAML

Pure Haskell YAML 1.2 processor

https://github.com/haskell-hvr/HsYAML

LTS Haskell 24.16:0.2.1.5@rev:1
Stackage Nightly 2025-10-24:0.2.1.5@rev:1
Latest on Hackage:0.2.1.5@rev:1

See all snapshots HsYAML appears in

GPL-2.0-only licensed by Herbert Valerio Riedel
Maintained by https://github.com/haskell-hvr/HsYAML
This version can be pinned in stack with:HsYAML-0.2.1.5@sha256:ca7cbb5bfdc89a9fc21e60642fafd4accf09ee489182513ae3790416147ab1d0,5407

Module documentation for 0.2.1.5

Used by 1 package in nightly-2025-10-24(full list with versions):
HsYAML-aeson

HsYAML is a YAML 1.2 processor, i.e. a library for parsing and serializing YAML documents.

Features of HsYAML include:

  • Pure Haskell implementation with small dependency footprint and emphasis on strict compliance with the YAML 1.2 specification.

  • Direct decoding to native Haskell types via (aeson-inspired) typeclass-based API (see Data.YAML).

  • Allows round-tripping while preserving ordering, anchors, and comments at Event-level.

  • Support for constructing custom YAML node graph representation (including support for cyclic YAML data structures).

  • Support for the standard (untyped) Failsafe, (strict) JSON, and (flexible) Core "schemas" providing implicit typing rules as defined in the YAML 1.2 specification (including support for user-defined custom schemas; see Data.YAML.Schema).

  • Support for emitting YAML using Failsafe, (strict) JSON, and (flexible) Core "schemas" (including support for user-defined custom encoding schemas; see Data.YAML.Schema).

  • Event-based API resembling LibYAML's Event-based API (see Data.YAML.Event).

  • Low-level API access to lexical token-based scanner (see Data.YAML.Token).

See also the HsYAML-aeson package which allows to decode and encode YAML by leveraging aeson's FromJSON and ToJSON instances.

Changes

0.2.1.5

2025-03-11, Andreas Abel

  • Drop support for old dependencies predating LTS 7.24 (GHC 8.0).
  • Relax bound on containers.
  • Tested with GHC 8.0 - 9.12.1.

0.2.1.4

2024-04-25, Andreas Abel

  • Drop support for GHC 7.
  • Testsuite: relax lower bounds to accommodate LTS 11.22 (GHC 8.2) for new Stack CI.
  • Tested with GHC 8.0 - 9.10.0 (alpha3).

0.2.1.3

2023-10-14, Andreas Abel

  • Pacify x-partial warning of GHC 9.8.
  • Tested with GHC 7.10 - 9.8.1.

0.2.1.2

2023-09-29, Andreas Abel

  • Add default-extensions: TypeOperators to silence warning under GHC ≥ 9.4.
  • Support latest versions of dependencies.
  • Tested with GHC 7.10 - 9.8.0.

0.2.1.1

2022-05-11, Andreas Abel

  • Compatibility with mtl-2.3.
  • Tested with GHC 7.4 - 9.2.

0.2.1.0

2019-10-06, Herbert Valerio Riedel

  • Define Functor Doc instance (#33)
  • New withScalar function and also define ToYAML Scalar and FromYAML Scalar instances
  • Export Pair type synonym from Data.YAML (#31)
  • New Data.YAML.prettyPosWithSource function for pretty-printing source locations (i.e. Pos values)
  • Add export docRoot :: Doc n -> n field accessor for convenience (#32)

0.2.0.0

This release incorporates the work from Vijay Tadikamalla’s GSOC 2019 Project. Highlights of this major release include support for emitting YAML as well as providing direct access to source locations throughout the parsing pipeline for improved error reporting.

  • Changes in Data.YAML module

    • YAML 1.2 Schema encoders (#21)
    • New ToYAML class for encoding Haskell Data-types from which YAML nodes can be constructed (#20)
      • New functions like encodeNode, encodeNode' for constructing AST
      • New functions like encode, encode1, encodeStrict, encode1Strict for supporting typeclass-based dumping
      • Some ToYAML instances and other api
    • Modify typeMismatch function to show error source location in error messages (#19)
    • Provide location-aware failAtNode alternative to fail

  • Changes in Data.YAML.Event module

    • Preserve and round-trip Comments at Event level(#24)
      • New Comment Event to preserve comments while parsing
      • Some additional implementations to preserve and round-trip comments
    • Fix issue #22
    • New EvPos type for recording event and their corresponding position (#19)
    • Preserve Flow Mapping and Flow sequence (#18)
    • Features to preserve Literal/Folded ScalarStyle (#15)
      • New Chomp type denoting Block Chomping Indicator
      • New IndentOfs type denoting Block Indentation Indicator
    • New NodeStyle type denoting flow/block style
    • Event(SequenceStart,MappingStart) constructors now record NodeStyle
    • Style type renamed to ScalarType
    • New writeEvents and writeEventsText function
    • Event(DocumentStart) now records YAML directive
    • Event parser now rejects duplicate/unsupported YAML/TAG directives as mandated by the YAML 1.2 specification

  • Move some schema related definitions from Data.YAML into the new Data.YAML.Schema module

  • Make decode, decode1, decodeStrict, decode1Strict, decodeNode, and decodeNode' treat duplicate keys (under the respective YAML schema) in YAML mappings as a loader-error (controllable via new schemaResolverMappingDuplicates schema property)

  • Define Generic and NFData instances for most types

  • Fix X38W testcase (#13, #14)

0.1.1.3

  • Fix bug in float regexp being too lax in the JSON and Core schema (#7)
  • Remove dependency on dlist

0.1.1.2

  • Tolerate BOM at each l-document-prefix (rather than only at the first one encountered in a YAML stream)
  • Workaround broken mtl-2.2.2 bundled in GHC 8.4.1 (#1)
  • Relax to GPL-2.0-or-later

0.1.1.1

  • Reject (illegal) non-scalar code-points in UTF-32 streams
  • Tolerate BOM at start of stream
  • Disambiguate choice in l-any-document production regarding token separation of c-directives-end
  • Fix c-indentation-indicator(n) grammar production when auto-detecting indentation in the presence of empty leading lines; also reject (illegal) auto-indent-level scalars with leading more-indented all-space lines
  • Complete character escape rules for double-quoted scalars
  • Minor optimizations

0.1.1.0

  • Data.YAML module promoted from TrustWorthy to Safe
  • Add FromYAML Natural instance
  • Add MonadFail, Alternative, and MonadPlus instances for Data.YAML.Parser
  • Add Data.YAML.decodeStrict function
  • Export Data.YAML.typeMismatch helper function

0.1.0.0

  • First version. Released on an unsuspecting world.