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Advanced Topic 44 of 50 Languages & Compilers

Hindley-Milner Inference

Classic algorithm for polymorphic type inference.

How it works

HM infers the most general polymorphic type by assigning type variables and unifying constraints (Algorithm W), underlying ML and Haskell.

Where it is used

Powerful inference without annotations.

Common pitfall

Higher-rank types and subtyping break complete inference.

Where it fits in the course

Hindley-Milner Inference is topic 44 of 50 in Languages & Compilers, rated Advanced. It follows Ownership and Borrowing. The next topic is Dependent Types.

Practice and explore

How to study this topic

  1. Read the explanation above and restate the definition in your own words.
  2. Open the interactive lesson and answer its mini quiz.
  3. Read the common pitfall, then write a short example that avoids it.
  4. Check your understanding in the Languages & Compilers quiz, and revisit the topic if you miss a question.

Frequently asked questions

What is Hindley-Milner Inference?
Classic algorithm for polymorphic type inference. It belongs to Languages & Compilers and is an advanced-level topic.
Where do people use Hindley-Milner Inference?
Powerful inference without annotations.
What is the most common mistake with Hindley-Milner Inference?
Higher-rank types and subtyping break complete inference.
What should I learn before this topic?
Start with Ownership and Borrowing, the topic just before it in Languages & Compilers.
How can I practice it?
Answer the mini quiz in the interactive lesson, then take the Languages & Compilers quiz.
Is Hindley-Milner Inference hard to learn?
It is an Advanced topic. Expect to read the explanation more than once and to connect it with earlier topics. Working through a small example by hand helps most.
How long does it take to learn Hindley-Milner Inference?
Most learners can read the explanation and finish the interactive lesson in under an hour. Fluency comes from applying the idea, so plan a few short practice sessions with the quiz.
Which topics are related to Hindley-Milner Inference?
Ownership and Borrowing, Dependent Types, Pattern Matching are closely connected. The full list is in the related topics section of this page.
What should I study after Hindley-Milner Inference?
Continue with Dependent Types, the next topic in Languages & Compilers.

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