postgresql-types-algebra

Core type classes and algebra for PostgreSQL type mappings, extracted from "postgresql-types".

This package provides the fundamental abstractions for mapping Haskell types to PostgreSQL types:

• Type classes defining the algebra for PostgreSQL type mappings
• Error types for decoding failures
• Metadata support for type names, OIDs, and parameters
• No concrete implementations - just the algebra

The core type class for types that map to PostgreSQL values:

class IsPrimitive a where
  -- Type metadata
  typeName :: Tagged a Text
  baseOid :: Tagged a (Maybe Word32)
  arrayOid :: Tagged a (Maybe Word32)
  typeParams :: Tagged a [Text]
  typeSignature :: Tagged a Text

  -- Textual format
  textualEncoder :: a -> TextBuilder.TextBuilder
  textualDecoder :: Attoparsec.Parser a

Evidence that a type additionally has a PostgreSQL binary wire format. Not every PostgreSQL type supports binary transmission - some only have textual send/receive functions registered on the server - so this is a separate, optional subclass rather than part of IsPrimitive itself:

class (IsPrimitive a) => IsBinaryPrimitive a where
  binaryEncoder :: a -> Write.Write
  binaryDecoder :: PtrPeeker.Variable (Either DecodingError a)

These classes enable:

• Binary encoding/decoding using PostgreSQL's native binary format, where supported
• Textual encoding/decoding using PostgreSQL's text representation
• Type metadata including OIDs and type signatures for parameterized types
• Round-trip fidelity through all encoding combinations

Use this package to:

• Define custom PostgreSQL type mappings compatible with the "postgresql-types" ecosystem
• Create adapter libraries for different PostgreSQL client libraries
• Build generic tools that work with any IsPrimitive (and, where applicable, IsBinaryPrimitive) instance

This package is typically used indirectly through:

• "postgresql-types" - Concrete type implementations
• "hasql-postgresql-types" - "hasql" integration
• "postgresql-simple-postgresql-types" - "postgresql-simple" integration