Default values of Haskell Types

Java types have default values. If you don’t explicitly assign a value in a field declaration, it gets its default value of the given type. For example, the default value of int is 0 and the default value of boolean is false.

On the contrary, Haskell types do not provide default values. This is natural because Haskell does not destructively update a variable initialized with the the default value.

However, it is handy to have a default value in some cases such as a record with many fields. Haskell libraries often provide a default value for such a record for the ease of construction. For example, llvm-general-pure provides defaultModule, which is the default value of Module. The data constructor Module has 4 fields:

• moduleName :: String
• moduleDataLayout :: Maybe DataLayout – a DataLayout, if specified, must match that of the eventual code generator
• moduleTargetTriple :: Maybe String
• moduleDefinitions :: [Definition]

Using defaultModule, you don’t need to supply all these fields. You can construct a Module value using Haskell record update syntax as in the following:

defaultModule { moduleName="mymodule" }

The data-default package provides a type class Default, which is useful for this purpose. If a given type is an instance of Default, you can get its default value using def method. Instances are provided for (), Set, Map, Int, Integer, Float, Double, and many others.

Prelude Data.Default> def :: Int
0
Prelude Data.Default> def :: [a]
[]
Prelude Data.Default> def :: Double
0.0