Data.Avro.Deriving

This module lets us derive Haskell types from an Avro schema that can be serialized/deserialzed to Avro.

Deriving options

data DeriveOptions

Derives Avro from a given schema file. Generates data types, FromAvro and ToAvro instances.

Constructors

DeriveOptions

Fields

• fieldNameBuilder :: Text -> Field -> Text

  How to build field names for generated data types. The first argument is the type name to use as a prefix, rendered according to the namespaceBehavior setting.

• fieldRepresentation :: TypeName -> Field -> (FieldStrictness, FieldUnpackedness)

  Determines field representation of generated data types

• namespaceBehavior :: NamespaceBehavior

  Controls how we handle namespaces when defining Haskell type and field names.

Instances

Generic DeriveOptions

Defined in Data.Avro.Deriving

Associated Types

type Rep DeriveOptions :: Type -> Type

Methods

from :: DeriveOptions -> Rep DeriveOptions x

to :: Rep DeriveOptions x -> DeriveOptions

type Rep DeriveOptions

Defined in Data.Avro.Deriving

type Rep DeriveOptions = D1 ('MetaData "DeriveOptions" "Data.Avro.Deriving" "avro-0.6.1.0-9WMp5t0QC4JDZvUJwEXo6V" 'False) (C1 ('MetaCons "DeriveOptions" 'PrefixI 'True) (S1 ('MetaSel ('Just "fieldNameBuilder") 'NoSourceUnpackedness 'NoSourceStrictness 'DecidedLazy) (Rec0 (Text -> Field -> Text)) :*: (S1 ('MetaSel ('Just "fieldRepresentation") 'NoSourceUnpackedness 'NoSourceStrictness 'DecidedLazy) (Rec0 (TypeName -> Field -> (FieldStrictness, FieldUnpackedness))) :*: S1 ('MetaSel ('Just "namespaceBehavior") 'NoSourceUnpackedness 'NoSourceStrictness 'DecidedLazy) (Rec0 NamespaceBehavior))))

data FieldStrictness

Describes the strictness of a field for a derived data type. The field will be derived as if it were written with a !.

Constructors

StrictField

LazyField

Instances

Generic FieldStrictness

Defined in Data.Avro.Deriving

Associated Types

type Rep FieldStrictness :: Type -> Type

Methods

from :: FieldStrictness -> Rep FieldStrictness x

to :: Rep FieldStrictness x -> FieldStrictness

type Rep FieldStrictness

Defined in Data.Avro.Deriving

type Rep FieldStrictness = D1 ('MetaData "FieldStrictness" "Data.Avro.Deriving" "avro-0.6.1.0-9WMp5t0QC4JDZvUJwEXo6V" 'False) (C1 ('MetaCons "StrictField" 'PrefixI 'False) (U1 :: Type -> Type) :+: C1 ('MetaCons "LazyField" 'PrefixI 'False) (U1 :: Type -> Type))

data FieldUnpackedness

Describes the representation of a field for a derived data type. The field will be derived as if it were written with an {--} pragma.

Constructors

UnpackedField

NonUnpackedField

Instances

Generic FieldUnpackedness

Defined in Data.Avro.Deriving

Associated Types

type Rep FieldUnpackedness :: Type -> Type

Methods

from :: FieldUnpackedness -> Rep FieldUnpackedness x

to :: Rep FieldUnpackedness x -> FieldUnpackedness

type Rep FieldUnpackedness

Defined in Data.Avro.Deriving

type Rep FieldUnpackedness = D1 ('MetaData "FieldUnpackedness" "Data.Avro.Deriving" "avro-0.6.1.0-9WMp5t0QC4JDZvUJwEXo6V" 'False) (C1 ('MetaCons "UnpackedField" 'PrefixI 'False) (U1 :: Type -> Type) :+: C1 ('MetaCons "NonUnpackedField" 'PrefixI 'False) (U1 :: Type -> Type))

data NamespaceBehavior

How to treat Avro namespaces in the generated Haskell types.

Constructors

IgnoreNamespaces

Namespaces are ignored completely. Haskell identifiers are generated from types' base names. This produces nicer types but fails on valid Avro schemas where the same base name occurs in different namespaces.

The Avro type com.example.Foo would generate the Haskell type Foo. If Foo had a field called bar, the generated Haskell record would have a field called fooBar.

HandleNamespaces

Haskell types and field names are generated with namespaces. See deriveAvroWithNamespaces for an example of how this works.

The Avro type com.example.Foo would generate the Haskell type ComexampleFoo. If Foo had a field called bar, the generated Haskell record would have the field comexampleFooBar.

Custom (Text -> [Text] -> Text)

Provide a custom mapping from the name of the Avro type and its namespace that will be used to generate Haskell types and fields.

defaultDeriveOptions :: DeriveOptions

Default deriving options

defaultDeriveOptions = DeriveOptions
  { fieldNameBuilder  = mkPrefixedFieldName
  , fieldStrictness   = mkLazyField
  , namespaceBehavior = IgnoreNamespaces
  }

mkPrefixedFieldName :: Text -> Field -> Text

Generates a field name that is prefixed with the type name.

For example, if the schema defines type Person that has a field firstName, then the generated Haskell type will be like

Person { personFirstName :: Text }

mkAsIsFieldName :: Text -> Field -> Text

Generates a field name that matches the field name in schema (sanitised for Haskell, so first letter is lower cased)

For example, if the schema defines type Person that has a field firstName, then the generated Haskell type will be like

Person { firstName :: Text }

You may want to enable DuplicateRecordFields if you want to use this method.

mkLazyField :: TypeName -> Field -> (FieldStrictness, FieldUnpackedness)

Marks any field as non-strict in the generated data types.

mkStrictPrimitiveField :: TypeName -> Field -> (FieldStrictness, FieldUnpackedness)

Make a field strict and unpacked if it has a primitive representation. Primitive types are types which GHC has either a static or an unlifted representation: (), Boolean, Int32, Int64, Float, Double.

Deriving Haskell types from Avro schema

makeSchema :: FilePath -> Q Exp

Generates the value of type Schema that it can later be used with deriveAvro' or deriveAvroWithOptions'.

mySchema :: Schema
mySchema = $(makeSchema "schemas/my-schema.avsc")

makeSchemaFrom :: FilePath -> Text -> Q Exp

makeSchemaFromByteString :: ByteString -> Q Exp

deriveAvroWithOptions :: DeriveOptions -> FilePath -> Q [Dec]

Derives Haskell types from the given Avro schema file. These Haskell types support both reading and writing to Avro.

For an Avro schema with a top-level record called com.example.Foo, this generates:

• a Schema with the name schema'Foo or schemacom'exampleFoo, depending on the namespaceBehavior setting.
• Haskell types for each named type defined in the schema
• HasSchema instances for each type
• FromAvro instances for each type
• ToAvro instances for each type

This function ignores namespaces when generated Haskell type and field names. This will fail on valid Avro schemas which contain types with the same base name in different namespaces. It will also fail for schemas that contain types with base names that are the same except for the capitalization of the first letter.

The type com.example.Foo will generate a Haskell type Foo. If com.example.Foo has a field named Bar, the field in the Haskell record will be called fooBar.

deriveAvroWithOptions' :: DeriveOptions -> Schema -> Q [Dec]

Derive Haskell types from the given Avro schema.

For an Avro schema with a top-level definition com.example.Foo, this generates:

• a Schema with the name schema'Foo or schemacom'exampleFoo depending on namespace handling
• Haskell types for each named type defined in the schema
• HasSchema instances for each type
• FromAvro instances for each type
• ToAvro instances for each type

deriveAvroFromByteString :: ByteString -> Q [Dec]

Same as deriveAvro but takes a ByteString rather than FilePath

deriveAvro :: FilePath -> Q [Dec]

Same as deriveAvroWithOptions but uses defaultDeriveOptions

deriveAvro = deriveAvroWithOptions defaultDeriveOptions

deriveAvro' :: Schema -> Q [Dec]

Same as deriveAvroWithOptions' but uses defaultDeriveOptions

deriveAvro' = deriveAvroWithOptions' defaultDeriveOptions

Re-exporting a quasiquoter for raw string literals

r :: QuasiQuoter

A quasiquoter for raw string literals - that is, string literals that don't recognise the standard escape sequences (such as '\n'). Basically, they make your code more readable by freeing you from the responsibility to escape backslashes. They are useful when working with regular expressions, DOS/Windows paths and markup languages (such as XML).

Don't forget the LANGUAGE QuasiQuotes pragma if you're using this module in your code.

Usage:

    ghci> :set -XQuasiQuotes
    ghci> import Text.RawString.QQ
    ghci> let s = [r|\w+@[a-zA-Z_]+?\.[a-zA-Z]{2,3}|]
    ghci> s
    "\\w+@[a-zA-Z_]+?\\.[a-zA-Z]{2,3}"
    ghci> [r|C:\Windows\SYSTEM|] ++ [r|\user32.dll|]
    "C:\\Windows\\SYSTEM\\user32.dll"

Multiline raw string literals are also supported:

    multiline :: String
    multiline = [r|<HTML>
    <HEAD>
    <TITLE>Auto-generated html formated source</TITLE>
    <META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=windows-1252">
    </HEAD>
    <BODY LINK="800080" BGCOLOR="#ffffff">
    <P> </P>
    <PRE>|]

Caveat: since the "|]" character sequence is used to terminate the quasiquotation, you can't use it inside the raw string literal. Use rQ if you want to embed that character sequence inside the raw string.

For more on raw strings, see e.g. http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2006/n2053.html

For more on quasiquotation, see http://www.haskell.org/haskellwiki/Quasiquotation