Language Design: Typing Terminology

Published on 2022-08-06. Last updated on 2025-06-12

Most people think only in terms of the dichotomy between Nominal-Manifest-Static-Strong and Structural-Inferred-Dynamic-Weak in any given discussion of programming language type system design. And it is exhausting.

(from what does strong and weak typing mean to you)

Most individual distinction are a scale, not a strict yes/no checkbox. (inspiration)

Typed ⟷ Untyped (Typing Modality/Presence)

A typed language allows analysis of the program text and rejection of certain programs, without executing them. Untyped (sometimes also called “dynamic”) programming languages lack this facility.

More Static: Haskell, CommonLisp
More Untyped: SmallTalk, Scheme, JavaScript

Manifest ⟷ Inferred (Typing Apparency)

It describes the degree to which types need to mentioned in the program text.

More Manifest: Java, C
More Inferred: OCaml, Haskell.

Nominal ⟷ Structural (Typing Morphology)

It pertains to how types are described and referred to and when they are judged equal.

More Nominal: Rust, D
More Structural: Ruby, OCaml.

→ Mention Java SAM types.

Reified ⟷ Erased (Typing Preservation)

A reified type system means that type information is preserved during compilation and available at run-time.

This enables runtimes to execute code more effectively and/or allows user-provided code to query type information as part of the program code.

Java: java.lang.reflect package, Object#getClass()
C#: Object#GetType()

Compile-time vs. Run-time Reflection?

~~ compile-time: required info is stored at call-site vs. run-time: required info is stored at declaration-site

Not directly typing related, but typing preservation choices have a direct impact on what’s possible. (Maybe as sub-point of reified vs. erased?)

vv Draft Space vv

Untyped Languages

Tagged ⟷ Untagged

→ cite Benjamin Pierce → cite Bob Harper: https://existentialtype.wordpress.com/2011/03/19/dynamic-languages-are-static-languages/

Not really typing:

Strong ⟷ Weak (Typing Discipline/Value Convertibility)

which I think should be called Typing discipline because it pertains to the number of type errors you get (compile time or runtime, doesn’t matter.) Stronger: SML, Python, Weaker: JavaScript, C.