Data.Char Module – Haskell

In Haskell, working with characters is common, especially when processing text or creating parsers. The Data.Char module is Haskell’s go-to library for character manipulation, offering a wide array of functions to classify, convert, and operate on characters. In this article, we’ll dive into the essentials of the Data.Char module, exploring its most useful functions and how they can be applied.

Why Use Data.Char?

The Data.Char module provides functions to handle individual characters (Char type), which are essential when dealing with text processing tasks. With Data.Char, you can easily check if a character is a digit, convert characters to uppercase, or even work with Unicode.

To use Data.Char, simply import it at the beginning of your Haskell file:

import Data.Char

Let’s look at some of the key functionalities in Data.Char.

1. Character Classification Functions

Character classification functions check if a character belongs to a specific category, like a digit, letter, or punctuation mark. Some of the commonly used classification functions are:

isAlpha: Checks if a character is an alphabetic letter (a-z, A-Z).

isAlpha 'a'  -- True
isAlpha '1'  -- False

isDigit: Checks if a character is a numeric digit (0-9).

isDigit '5'  -- True
isDigit 'a'  -- False

isSpace: Checks if a character is whitespace (space, tab, newline, etc.).

isSpace ' '  -- True
isSpace 'a'  -- False

isUpper and isLower: Checks if a character is uppercase or lowercase, respectively.

isUpper 'A'  -- True
isLower 'a'  -- True

These functions are helpful when you need to validate or filter specific character types in strings.

2. Character Conversion Functions

Data.Char offers functions to convert characters between uppercase and lowercase, as well as to convert characters to their Unicode code points.

toUpper: Converts a character to uppercase.

toUpper 'a'  -- 'A'

toLower: Converts a character to lowercase.

toLower 'A'  -- 'a'

digitToInt: Converts a numeric character (0-9) to an integer.

digitToInt '5'  -- 5

intToDigit: Converts an integer (0-9) to its corresponding character.

intToDigit 5  -- '5'

Using these functions, you can transform text case or convert characters to integers and vice versa.

3. Unicode and ASCII

Haskell characters are represented by Unicode, making the Data.Char module compatible with international characters beyond ASCII. Here are some functions that help with Unicode manipulation:

ord: Returns the Unicode code point of a character.

ord 'A'  -- 65
ord 'あ'  -- 12354

chr: Converts an integer code point to its corresponding character.

chr 65  -- 'A'
chr 12354  -- 'あ'

These functions are essential if you’re working directly with Unicode values, enabling encoding and decoding between characters and numeric representations.

4. Alphabetical and Numerical Ranges

The Data.Char module allows for easy creation of character ranges using enumFromTo syntax or basic range notation. This can be helpful when generating sequences of alphabetic or numeric characters:

Example: Generate a list of lowercase letters.

['a' .. 'z']  -- "abcdefghijklmnopqrstuvwxyz"

Example: Generate a list of digits.

['0' .. '9']  -- "0123456789"

This is a powerful way to handle lists of characters without manually specifying each character.

5. Using Data.Char for Text Transformation

Let’s see a few examples of how the Data.Char module can be applied in practical text transformation tasks.

Example 1: Converting Text to Title Case

You can use Data.Char to capitalize the first letter of each word in a string:

import Data.Char (toUpper, toLower)

toTitleCase :: String -> String
toTitleCase [] = []
toTitleCase (x:xs) = toUpper x : map toLower xs

Example 2: Filtering Digits from a String

Suppose you want to extract only the digits from a string. Here’s how you can do it:

import Data.Char (isDigit)

extractDigits :: String -> String
extractDigits = filter isDigit

Example 3: Validating an Alphabetic String

You can check if a string contains only alphabetic characters with isAlpha.

import Data.Char (isAlpha)

isAlphabetic :: String -> Bool
isAlphabetic = all isAlpha

6. More Useful Functions

Data.Char also includes several other useful functions:

generalCategory: Returns the Unicode category of a character (e.g., UppercaseLetter, Space, DecimalNumber).

generalCategory 'A'  -- Result: UppercaseLetter

isControl: Checks if a character is a control character (e.g., newline or tab).

isControl '\n'  -- Result: True

Conclusion

The Data.Char module is a powerful and versatile library for character manipulation in Haskell. From basic classification functions to Unicode handling and transformations, it provides the tools you need for a variety of text processing tasks. Whether you’re validating input, transforming text, or simply working with individual characters, Data.Char offers the functionality to handle characters efficiently.

With a solid understanding of Data.Char, you can streamline character-based operations in Haskell, enhancing the readability and maintainability of your code.

💡 Helpful References

Haskell.org - Data.List
https://hackage.haskell.org/package/base-4.20.0.1/docs/Data-Char.html