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ADL haskell backend

Usage: adlc haskell [OPTION...] files...
  -I DIR  --searchdir=DIR      Add the specifed directory to the ADL searchpath
  -O DIR  --outputdir=DIR      Set the directory where generated code is written
          --merge-adlext=EXT   Add the specifed adl file extension to merged on loading
          --verbose            Print extra diagnostic information, especially about files being read/written
          --no-overwrite       Don't update files that haven't changed
          --package=PACKAGE    The language package into which the generated ADL code will be placed
          --include-rt         Generate the runtime code
          --rtpackage=PACKAGE  The haskell package where the ADL runtime is located

Generated Code

The haskell backend generates haskell code from the input ADL files. Each ADL module results in a single corresponding haskell module. The --package compiler flag specifies the root package for generate code. Hence, an adl module with the compile flag --package project1.adl would result in the haskell module

ADL structs and unions:

    struct Rectangle
        Double width;
        Double height;

    union Picture
        Circle circle;
        Rectangle rectangle;
        Vector<Picture> composed;
        Translated<Picture> translated;

produce haskell data definitions:

data Rectangle = Rectangle
    { rectangle_width :: Prelude.Double
    , rectangle_height :: Prelude.Double
    deriving (Prelude.Eq,Prelude.Ord,Prelude.Show)

data Picture
    = Picture_circle Circle
    | Picture_rectangle Rectangle
    | Picture_composed [Picture]
    | Picture_translated (Translated Picture)
    deriving (Prelude.Eq,Prelude.Ord,Prelude.Show)

Note that the constructors and field declarations are prefixed with the type name, to avoid conflicts with other declarations in the same module. The standard typeclasses Eq, Ord and Show are derived when possible.

ADL newtypes and type alias produce corresponding haskell definitions in the obvious way.

Every generated haskell type is also an instance of the AdlValue typeclass, primarily to support json parsing:

class AdlValue a where
  -- | A text string describing the type.
  atype :: Proxy a -> T.Text

  -- | A JSON generator for this ADL type
  jsonGen :: JsonGen a

  -- | A JSON parser for this ADL type
  jsonParser :: JsonParser a

Primitive Types

The ADL primitive types are mapped to haskell types as follows:

ADL Type Haskell Type
Int8,Int16,Int32,Int64 Int8,Int16,Int32,Int64
Word8,Word16,Word32,Word64 Word8,Word16,Word32,Word64
Bool Bool
Void ()
Float,Double Float,Double
String Data.Text
ByteVector Data.ByteString
Vector<T> [t]
StringMap<T> Data.Map Data.Text t
Nullable<T> Maybe t


The generated code depends upon a small runtime. The location of the runtime in the haskell module hierarchy can be controlled with the --rtpackage compiler flag. As a convenience, when the --include-rt flag is specified, the adl compiler will also output the runtime code.

As a concrete example, if the adl compiler is called like this:

adlc haskell\
  --outputdir src \
  --package adl \
  --rtpackage adl/runtime \
  --include-rt \

The following files will be created:


The runtime itself depends on the following haskell packages:

  • aeson
  • base
  • base64-bytestring
  • bytestring
  • containers
  • scientific
  • text
  • unordered-containers
  • vector

ie the transitive dependencies of the aeson package.


The haskell backend merges annotations from files with an .adl-hs suffix: eg when loading demo/model.adl it will automatically merge demo/model.adl-hs if found.

In order to avoid naming conflicts, the adl compiler generates haskell data definitions with a prefix for each field and constructor. As shown in the example above the default prefix is the name of the type with an underscore suffix (see the Picture and Rectangle examples above. The [HaskellFieldPrefix][] can be applied to declarations or individual fields to customize this prefix.


Custom types

When a type is defined in ADL, a (language independent) serialisation specification is implied. Running the compiler with a given language target will generate definitions for that type, as well the necessary serialisation code. However, often one would like to use an equivalent existing type, for compatibility with other code. Custom types make this possible.

As an example, consider a date type. There is no primitive in the ADL language for dates, so we need to define one. A possible ADL definition is:

newtype Date = String;   // dates are serialised as ISO-8601 strings.

This says that a Date is a new independent type, isomorphic to a string. This would normally result in the following generated haskell code:

newtype Date = Date { unDate :: T.Text }
    deriving (Prelude.Eq,Prelude.Ord,Prelude.Show)

instance ADLValue Date where ...

The issue here is that all user written code making use of ADL values must be prepared to convert from date strings to more appropriate native types. A solution is to use a custom type mapping to map the ADL Date definition to the Day type from the standard library. This means that, in all generated haskell code, Day will be used in lieu of the Date declaration shown above. The developer no longer need to write any conversion code - all dates throughout the ADL system use the library type of her choice.

A key thing to realise here is that, even when custom type mappings are being used, the ADL definitions fix the serialisation of the data types. This is what permits interoperability between processes coded in different languages.

Custom types are specified through ADL annotations. To integrate an arbitrary type into ADL, once needs to provide:

  • an AdlValue instance declaration
  • a constructor function (or functions, for a union)

A suitable haskell declaration for such a Date type can be found in the unit tests.

Then, an annotation tells the ADL compiler to subsitute this custom definition in place of the automatically generated one:

import adlc.config.haskell.HaskellCustomType;

newtype Date = String;   // dates are serialised as ISO-8601 strings.

annotation Date HaskellCustomType {
    "haskellname" : "Date",
    "haskellimports" : [
        "qualified Date"
    "insertCode" : [
        "type Date = Date.Date"
    "structConstructor" : "Date.fromText"

Standard Custom Types

There are predefined haskell custom type mappings for the following declarations in the adl standard library:

ADL Type Haskell Type
sys.types.Pair<T1,T2> (,)
sys.types.Map<K,V> Data.Map.Map
sys.types.Set Data.Set.Set
sys.types.Maybe prelude.Maybe
sys.types.Either<T1,T> prelude.Either