wireform-delta

wireform-delta implements a Delta Lake transaction log reader. Delta Lake stores table state as an append-only JSON commit log under _delta_log/, with periodic Parquet checkpoints to avoid replaying every commit. Use this package when you need to discover active data files, inspect table history, or time-travel to a specific table version from Haskell.

Key features

• JSON commit replay via Delta.Log action parsers
• Checkpoint Parquet reader that reconstructs add/remove/metadata actions
• Schema parsing from metaData actions
• Time travel to any committed version
• Version history matching the DESCRIBE HISTORY surface
• Partition tracking with partitionValues on active files
• Interop-tested against delta-rs

Basic usage

Open a Delta table at its latest version and list the active data files:

import qualified Delta.IO as DT
import qualified Data.Text as T

listActiveFiles :: FilePath -> IO ()
listActiveFiles tableRoot = do
  result <- DT.openDeltaTable tableRoot
  case result of
    Left err ->
      putStrLn err
    Right table -> do
      mapM_ (putStrLn . T.unpack) (DT.activeFilePaths table)
      putStrLn $
        "version="
          ++ show (DT.dtVersion table)
          ++ " active files="
          ++ show (length (DT.activeFilePaths table))

Time-travel to a specific version and inspect commit history:

inspectHistory :: FilePath -> IO ()
inspectHistory tableRoot = do
  latest <- DT.openDeltaTable tableRoot
  case latest of
    Left err ->
      putStrLn err
    Right table -> do
      history <- DT.historyEntries table
      mapM_ print history
      atV2 <- DT.openDeltaTableAt tableRoot 2
      case atV2 of
        Left err ->
          putStrLn err
        Right snap ->
          putStrLn $
            "files at v2: "
              ++ show (length (DT.activeFilePaths snap))

For lower-level control, parse individual commit lines with parseLogLine from Delta.Log, or read a checkpoint directly through Delta.Checkpoint.

Notable modules

Module	Purpose
Delta.Log	Commit action types, parseLogLine, snapshotFromActions
Delta.Checkpoint	Parquet checkpoint decoder
Delta.IO	openDeltaTable, openDeltaTableAt, historyEntries
Delta.Log (re-exported)	TableSnapshot, applyAction, schema helpers

Reading algorithm

openDeltaTable follows the Delta protocol:

1. Read _delta_log/_last_checkpoint if present.
2. Load the referenced checkpoint Parquet file.
3. Replay JSON commits with version numbers above the checkpoint.
4. Return the resulting DeltaTable snapshot.

If no checkpoint exists, the reader replays the full commit log from version 0.

Interop

The probe suite cross-checks active files, partition values, checkpoint reconstruction, and time-travel versions against delta-rs on fixtures from real Delta tables.