Data.Derive: A User Manual

by Neil Mitchell

Data.Derive is a library and a tool for deriving instances for Haskell programs. It is designed to work with custom derivations, SYB and Template Haskell mechanisms. The tool requires GHC, but the generated code is portable to all compilers. We see this tool as a competitor to DrIFT.

This document proceeds as follows:

  1. Obtaining and Installing Data.Derive
  2. Supported Derivations
  3. Using the Derive Program
  4. Using Template Haskell Derivations
  5. Writing a New Derivation


Thanks to everyone who has submitted patches and given assistance, including: Twan van Laarhoven, Spencer Janssen, Andrea Vezzosi, Samuel Bronson, Joel Raymont, Benedikt Huber, Stefan O'Rear.

Obtaining and Installing Data.Derive

Data.Derive is available using darcs:

darcs get --partial

Install the program using the standard sequence of Cabal magic:

cabal update
cabal install derive

Supported Derivations

Data.Derive is not limited to any prebuild set of derivations, see later for how to add your own. Out of the box, we provide instances for the following libraries.

  • Arbitrary - from the library QuickCheck
  • ArbitraryOld - from the library QuickCheck-
  • Arities - from the library derive
  • Binary - from the library binary
  • BinaryDefer - from the library binarydefer
  • Bounded - from the library base
  • Data - from the library base
  • Default - from the library derive
  • Enum - from the library base
  • EnumCyclic - from the library base
  • Eq - from the library base
  • Fold
  • Foldable - from the library base
  • From
  • Functor - from the library base
  • Has
  • Is
  • LazySet
  • Monoid - from the library base
  • NFData - from the library parallel
  • Ord - from the library base
  • PlateTypeable - from the library uniplate
  • Read - from the library base
  • Ref
  • Serial - from the library smallcheck
  • Set
  • Show - from the library base
  • Traversable - from the library base
  • Typeable - from the library base
  • Update
  • Using the Derive program

    Let's imagine we've defined a data type:

    data Color = RGB Int Int Int
               | CMYK Int Int Int Int
               deriving (Eq, Show)

    Now we wish to extend this to derive Binary and change to defining Eq using our library. To do this we simply add to the deriving clause.

    data Color = RGB Int Int Int
               | CMYK Int Int Int Int
               deriving (Show {-! Eq, Binary !-})

    Now running derive on the program containing this code will generate appropriate instances. How do you combine these instances back into the code? There are various mechanisms supported.

    Appending to the module

    One way is to append the text to the bottom of the module, this can be done by passing the --append flag. If this is done, Derive will generate the required instances and place them at the bottom of the file, along with a checksum. Do not modify these instances.

    Using CPP

    One way is to use CPP. Ensure your compiler is set up for compiling with the C Pre Processor. For example:

    {-# OPTIONS_GHC -cpp #-}
    {-# OPTIONS_DERIVE --output=file.h #-}
    module ModuleName where
    #include "file.h"

    Side-by-side Modules

    If you had Colour.Type, and wished to place the Binary instance in Colour.Binary, this can be done with:

    {-# OPTIONS_DERIVE --output=Binary.hs --module=Colour.Binary --import #-}

    Here you ask for the output to go to a particular file, give a specific module name and import this module. This will only work if the data structure is exported non-abstractly.

    Using Template Haskell Derivations

    One of Derive's major advantages over DrIFT is support for the Template Haskell (henceforth abbreviated "TH") system. This allows Derive to be invoked automatically during the compilation process, and (because it occurs with full access to the renamer tables) transparently supports deriving across module boundaries. The main disadvantage of TH-based deriving is that it is only portable to compilers that support TH; currently that is GHC only.

    To use the TH deriving system, with the same example as before:

    {-# LANGUAGE TemplateHaskell #-}
    import Data.DeriveTH
    import Data.Binary
    data Color = RGB Int Int Int
               | CMYK Int Int Int Int
               deriving (Show)
    $( derive makeEq ''Color )
    $( derive makeBinary ''Color )

    We need to tell the compiler to insert the instance using the TH splice construct, $( ... ) (the spaces are optional). The splice causes the compiler to run the function derive (exported from Data.DeriveTH), passing arguments makeFooBar and ''Color. The second argument deserves more explanation; it is a quoted symbol, somewhat like a quoted symbol in Lisp and with deliberately similar syntax. (Two apostrophes are used to specify that this name is to be resolved as a type constructor; just 'Color would look for a data constructor named Color.)

    Writing a New Derivation

    There are two methods for writing a new derivation, guessing or coding. The guessing method is substantially easier if it will work for you, but is limited to derivations with the following properties:

    If however your instance does meet these properties, you can use derivation by guess. Many instances do meet these conditions, for examples see: Eq, Ord, Data, Serial etc. If however you need to code the derivation manually see examples such as Update and Functor.