Lesson 17 of 55
9 mins readFunction Composition (.) & Application ($)
In Plain English
Compose functions mathematically with the dot operator (.): (f . g) x = f (g x). Use the low-precedence application operator ($) to eliminate cluttering parentheses.
Deep Dive: How It Works
Composition (.): (f . g) x evaluates g x first, then feeds the result to f.
Application ($): f $ x is simply f x, but $ has the lowest possible precedence, meaning putStrLn $ "Hello " ++ "World" avoids surrounding parentheses.
Point-Free Style: Writing functions without mentioning the input variable names (e.g. fn = sum . map (^2) . filter even).
Core Rules to Remember
Mathematical Composition (.): Chains pipeline stages together into a single new function.
Dollar Operator ($): Replaces trailing closing parentheses for clean readable code.
Live Interactive Example
Hit Run Code to see it livePoint-Free Composition and Dollar Operator
Python 3.12
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Output Console
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How it works: sumEvenSquares filters evens, squares them, and sums them in a point-free pipeline.
Your Turn: Micro Challenge
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Micro Exercise
Compose Double and Successor
- Define `f = (* 2) . (+ 1)`.
- Evaluate `f 4` and print `"Result: "` followed by `show (f 4)`.
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Sandbox Output
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