How deeply does Learn Haskell by Example cover newtype idiom?

Defines syntax, constructor hiding, and implementation patterns, but defers technical differences between newtype and data to an appendix.

Newtype idiom

As explained in Learn Haskell by Example

newtype is Haskell syntax for defining a distinct type with exactly one constructor containing exactly one field.

Use newtype when a value needs a distinct type around exactly one field. An AssocMap can wrap a list of key-value pairs while separating its representation from the public API. section 5.2.2

Pair the idiom with a module export list: export AssocMap without its constructor, but expose operations such as member, alter, empty, delete, and insert. This prevents callers from constructing invalid duplicate-key maps directly. section 5.2.2

A protected associative map

module Data.AssocMap (AssocMap, member, alter) where newtype AssocMap k v = AssocMap [(k, v)] member :: Eq k => k -> AssocMap k v -> Bool member key (AssocMap xs) = member' key xs empty :: AssocMap k v empty = AssocMap []

How Learn Haskell by Example covers it

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Also explained in

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Write Powerful Rust MacrosA newtype is a distinct wrapper type around an existing value that gives the value stronger type-level meaning and can enforce construction invariants.
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Functional Design and Architecture: Examples in HaskellA newtype is a distinct type wrapper around an existing representation that preserves its runtime representation while enforcing compile-time type distinctions.
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Idiomatic RustThe newtype pattern defines a distinct type, typically as a tuple struct containing an existing type, to add type information or behavior with minimal encapsulation.

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Related concepts

orphan ruleaddresses · Rust's orphan rule restricts trait implementations so that either the trait or the implementing type must be local to the defining crate.
type systemuses · A type system is a programming language’s set of rules and mechanisms for classifying values and expressions and constraining their valid uses and combinations.
type aliascontrasts with · A type alias is a declaration that gives an existing type an alternative name without creating a distinct type or representation.
tuple structuses · A tuple struct is a Rust struct whose unnamed fields are accessed by positional indices.
tupleuses · A tuple is a finite, ordered collection of elements treated as a single composite value or record.
structimplements · A struct is a composite data type or structure that groups related fields or values into a single unit.

This explainer was generated by Manning's knowledge graph from the text of Learn Haskell by Example. The section links point to the parts of the book it draws on.