Polymorphic Function Types in Scala 3

on YouTube ↗42m

" summary

Explains concrete Scala 3 changes: adds polymorphic function types that require a polymorphic apply method, implemented as a refinement trait and compiled by erasure that erases to FunctionN (uses FunctionXXL for >22 args) with casts inserted. Shows real uses: Tuple.map preserving element types, Shapeless generic folds for typeclass derivation, type-level sized lists and a Cats Effect example for a safe compute callback. Covers compiler work, SIP-49 experimental status, and proposed ergonomics like type-parameter closure and improved subtyping.

AI-written summary · about

" chapters (10)

1. 0:00Intro
2. 3:15Method references
3. 6:49Example usecase
4. 10:49Taking a step back
5. 13:29Type erasure (2/2)
6. 19:30Example 2: Preserving type information
7. 25:21Encapsulation
8. 29:33Polymorphic eta-expansion (1/2)
9. 34:38Idea 2: type parameter clause inference + eta-expansion
10. 36:43Better subtyping

" related talks

• 40mPolymorphic Function Types in Scala 3Guillaume Martres · Scala Days 2023 Madrid
• 32mUnderstanding the Type System of Scala 3Bill Venners · Scala Love in the City 2021
• 45mHow to be polymorphic in ScalaYuriy Polyulya · JavaDay 2013
• 38mThinking in Scala 3 TypesBill Venners, Frank Sommers · Scala Days 2025 Lausanne
• 49mType level Programming in ScalaMatt Bovel · ScalaCon 2022
• 32mAdvanced Features of Scala 3Bill Venners · Scale By the Bay 2021
• 43mAdding kind-polymorphism to the Scala programming languageMiles Sabin · Code Mesh LDN 2018
• 51mAdding Kind-polymorphism To The Scala Programming LanguageMiles Sabin · Scala Italy 2018