A Haskell module is a source file that groups related definitions. Its declaration names the module, and that name normally follows the file path: `Graph.hs` defines `Graph`, while `Foo/Bar/Module.hs` defines `Foo.Bar.Module`. Modules make a project easier to divide into focused pieces of library code and executable code. section 2.1.3 section 5.1.2 section 2.1.2
Export a small interface
An export list after the module name chooses what other modules can use. Here, callers can use `square` but do not automatically receive every definition in the file. `module Lib (square) where` `square :: Int -> Int` `square x = x * x` section 2.1.3
Use `import` to bring another module’s exported definitions into scope. Importing without restrictions brings in its whole export list. When names clash, qualify a use with its module name, such as `ModuleName.x`; clashes become more likely as a module imports more code. section 2.1.3 section 5.3
Separate the entry point from reusable code
A typical executable has `Main` as its entry-point module. It imports a library module, then its `main` action calls an exported definition. This keeps reusable functionality in `src` while `app` starts the program. `module Main where` `import Lib` `main :: IO ()` `main = someFunc` section 2.1.3 section 2.1.2
Use module boundaries to shape a project’s public interface:
- Put definitions for one concern together, such as graph-related functions in `Graph` or line-numbering types and transformations in a library module. section 5.1.2 section 4.4.2
- Export the types and functions that form the intended interface, while keeping implementation details out of client modules. section 4.4.2
- Keep pure core functionality in a library module and use it from an executable module that performs input and output. section 4.4.2
- Avoid tangled dependencies: name conflicts and cyclical imports can prevent compilation. section 7.4.2
Listing 7.15 A re-export of modules
module Csv
( module Csv.Conversion, #1
module Csv.Types,
)
where
import Csv.Conversion #2
import Csv.Types
A top-level module can re-export several focused modules, giving clients one import for related functionality while preserving the internal directory structure. section 7.4.2
