Lesson 01 of 55

8 mins read

Introduction to Haskell & GHC

In Plain English

Haskell is a purely functional, statically typed language where functions have no side effects, variables are immutable by default, and expressions are evaluated only when needed (lazy evaluation).

The Mental Model

"Think of pure functions in Haskell like mathematical formulas: f(x) = x^2 + 1 always yields the exact same output for the same input, with zero side effects or hidden mutable state."

Deep Dive: How It Works

Purity & Immutability: Every function is pure; given the same inputs, it always produces the same output without modifying global state or hardware registers behind the scenes.

Lazy Evaluation: Expressions are not evaluated when created; instead, they are represented as suspended computations (thunks) and evaluated only when their values are demanded.

GHC & GHCi: The Glasgow Haskell Compiler (GHC) compiles Haskell to native machine code; GHCi provides an interactive REPL.

Core Rules to Remember

Pure Functional Paradigm: Functions are first-class citizens without hidden mutations or state side effects.

Strong Static Typing: The compiler catches bugs before runtime through robust Hindley-Milner type inference.

Lazy Evaluation: Computes values only when required, enabling infinite data structures.

Common Beginner Traps & How to Fix Them

Mistake:Attempting to reassign a variable like in imperative languages (e.g. x = x + 1).

Why it happens: In Haskell, = defines a mathematical identity, not a stateful memory assignment.

How to fix: Define new values or use recursion with function arguments.

Pro Tip: In GHCi REPL, use :t expr to inspect the inferred type of any expression.

Live Interactive Example

Hit Run Code to see it live

First Haskell Program

Python 3.12

Output Console

Click "Run Code" to view the rendered output.

How it works: main is the entry point of a Haskell program with type IO (). putStrLn prints strings followed by newline.

Your Turn: Micro Challenge

No pressure! Edit the starter code below and test your solution with instant feedback.

Micro Exercise

Print a Greeting Message

Write `main = putStrLn "Hello, Haskell World!"`.

Sandbox Output

Click "Run & Check" to test your solution.

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