slackblocks for Haskell

Typed, immutable Slack Block Kit composition with checked construction and Aeson encoding. Requires GHC 9.6.7 or newer within the tested compiler range.

The native API covers all 89 shared models, closed roles and enumerations in contract 1.5.0. Haskell joins the coordinated 3.1.0 release on Hackage. Add slackblocks >= 3.2 && < 3.3 to your application's Cabal build-depends, run cabal update, then cabal build. The source archive and native HTTP integration are checked on supported compiler endpoints.

{-# LANGUAGE OverloadedStrings #-}

import Data.Aeson (encode)
import Slackblocks

example = do
  label <- buildPlainText (plainText "Open")
  action <-
    buildButtonElement
      ((buttonElement label) {buttonElementActionId = Just "open"})
  body <- buildMarkdownText (markdownText "*Hello*")
  section <-
    buildSectionBlock
      ( sectionBlock
          { sectionBlockText = Just (markdownTextAsText body),
            sectionBlockAccessory = Just (buttonElementAsElement action)
          }
      )
  buildMessagePayload
    ( (messagePayload "C123")
        { messagePayloadBlocks = Just [sectionBlockAsBlock section]
        }
    )

-- Handle Left errors or send encode payload on Right payload.

Field records are construction inputs, not wire values: they have no ToJSON instance. Checked values have hidden constructors. Inspect a value, edit its fields, then rebuild to validate the change. Optional Nothing omits a field; explicit Just False and empty text remain present where the contract allows them. Block IDs are omitted unless supplied. Validation reports a category, structured path, constraint and explanation without throwing exceptions.

fromValue and decode check JSON at the ingress boundary. decode also rejects duplicate object keys (including escaped spellings and opaque metadata), invalid UTF-8 and trailing input. Aeson's FromJSON instances validate models too, but cannot recover duplicate keys already collapsed by an external Aeson parser. The byte decoder also rejects numeric exponent overflow before Aeson can narrow an exponent to Int. Checked native values and JSON ingress reject numbers whose normalized encoding would overflow or fail to round-trip, including numbers in metadata and extensions. This is a Scientific representation boundary, not a Slack magnitude limit; decimal bounds supplied as strings keep their arbitrary-exponent support. Already parsed Aeson values cannot recover an original exponent that an external parser truncated. Integer fields with shared bounds are range-checked before expanding exponents, so oversized inputs fail without first allocating an enormous integer. Use Slackblocks.decode when accepting untrusted JSON bytes. Typed child roles reject unsupported elements, while metadata and explicit unmodeled extension fields preserve arbitrary JSON. Extensions cannot override modeled fields or the type discriminator. Unknown enum values and receiving-element style flags are rejected. JSON null is treated as absence for an optional field and as a missing value for a required field.

Use buildPlanTask (taskCardBlock id title TaskStatusPending) for a pending task inside a plan. buildTaskCardBlock checks standalone cards and rejects pending. A PlanTask cannot be placed in a message's block list. Number-input bounds are decimal strings; decimalText converts Scientific values without going through a binary floating-point number. Exponents are compared without expanding them into potentially enormous integers.

Run cabal test all from the package directory. Tests use local data and make no Slack requests. The package version is the coordinated 3.2.0 release; the independent shared contract version is 1.5.0.

The conformance suite constructs all 120 valid fixtures through native APIs and independently checks their wire objects. It also checks every invalid input at JSON ingress, native invalid constructions where representable, and compiler rejections where the type system prevents construction. python bin/check adds generated-source drift, bundled-spec parity, compilation boundaries, cabal check, a source archive and a fresh archive consumer. Install the source tools separately with cabal install ormolu-0.7.7.0 --ignore-project and cabal install hlint-3.8 --ignore-project before running the full checker.

The full checker requires deterministic formatting and generation, HLint, independent compiled export discovery and at least 95% handwritten expression coverage across validation, wire handling, strict JSON, decimal comparison, style and helpers. HPC reports branches and declarations separately; derived Eq/Show declarations do not stand in for behavior tests. The two generated modules remain in native/conformance and compiled API checks, but are excluded from the handwritten percentage. All other production modules are discovered from source and compiled coverage receipts; missing modules fail the gate. Negative controls prove that unclassified exports and missing/zero coverage fail. Five randomized properties each run 500 cases with a recorded reproducible seed.

accordionSection returns editable container fields; build each section and use accordion to check and preserve their order. paginate uses an explicit action ID prefix and page settings, adds previous/next controls and an optional indicator, and leaves expanded surface limits to checked payload construction. It does not send messages or store page state. workflowFromURL, named/hex/RGB colors and blockKitBuilderURL are pure helpers. Builder links accept checked payloads or an explicit BuilderJSON object; they do not make network requests.

The supported compiler series are GHC 9.6 through 9.14. Compiler requirements belong to this package, not to unrelated Slack clients:

Cabal 3.16.1.0 is the pinned development tool. The package uses Cabal syntax 3.0 and GHC2021. Tested lower dependency bounds are Aeson 2.2.0.0, bytestring 0.11.5.4, text 2.0.2, containers 0.6.7, scientific 0.3.7.0 and vector 0.13.0.0. QuickCheck is test-only. Exact lower, committed locked and latest-within-bounds resolutions run independently; both Aeson ordered and unordered KeyMap modes execute every native suite. The library does not impose its development lock on downstream consumers. Resolution receipts record actual versions and flags.

python bin/package-check extracts the actual source distribution outside the checkout, executes every bundled suite, builds Haddock, then builds two fresh consumers against the tarball. Neither consumer has a workspace override or access to the monorepo's spec. One checks the public API, encoding, validation and editing; the other checks real HTTP request bytes against a local server. Run python bin/dependency-check locked, lower, or latest to inspect a specific dependency profile; --unordered-keymap selects the alternative Aeson representation. lower uses GHC 9.6.7 and locked has endpoint compiler profiles.

The separate same-commit quality job owns source tools, export discovery and HPC. The aggregate native status requires native, quality, dependency, intermediate compiler and sending jobs. The default local python bin/check executes all applicable profiles and archive/transport checks as well as the quality gates.

For native sending, see the runnable HTTP integration. HTTP and TLS packages remain outside the library's dependencies. This is an application example, not a credential store or an automatic retry service.