Module Management in Haskell

1. Module Management and Interface
2. Module Architecture
3. Importing Module

1. Module Management and Interface

We can control what to expose in the brackets after module name. For example,

  module ModuleName ( export1, export2 ) where

2. Module Architecture

Module names usually have their first letter capitalized, e.g., Data.List. The dots in the module names refer to folder architecture.

The root directory of modules can be configured in cabal as hs-source-dirs: src.

3. Importing Module

Suppose in Module, we have functions foo, bar and datatype Maybe with constrcutors Just, Nothing.

3.1. Import to Global Namespace

We can directly import, this will import all symbols to the global namespace, which may cause issues.

  import Module

  -- Then we can use directly
  foo ...

3.2. Import Namespaced

We can import with namespace isolation, using qualified keyword.

  import qualified Module

  -- Then we can invoke with namespace
  Module.foo

3.3. Import with Aliases

Even with qualified, we can use as keyword to give it an alias for convenience.

  import qualified Module as M

  -- Then we can invoke functions with alias
  M.foo

3.4. Selective Import

We can import only part of the module.

  import Module (foo)

  -- we can only use foo, not bar
  foo ...

Notably, if we want to selective import a type/typeclass, we have to import all constructors.

  import Module (Maybe(..)) -- use .. to import all data constructors

Of course, we can choose not to expose any constructors or expose part of the constructors, so as to implement abstract data type to provide data security.