postgresql-types-algebra

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

Overview

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

Type Classes

IsScalar

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

class IsScalar 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
  
  -- Binary format
  binaryEncoder :: a -> Write.Write
  binaryDecoder :: PtrPeeker.Variable (Either DecodingError a)
  
  -- Textual format
  textualEncoder :: a -> TextBuilder.TextBuilder
  textualDecoder :: Attoparsec.Parser a

This class enables:

• Binary encoding/decoding using PostgreSQL's native binary format
• 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 Cases

For Library Authors

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 IsScalar instance

For Application Developers

This package is typically used indirectly through:

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