042: Comparing TypeScript and Elm's Type Systems

• TypeScript and Elm have different goals
• Soundness is not a goal of the TypeScript type system
• TypeScript Design Goals (and non-goals)
• TypeScript's any type
• Nominal vs structural typing

TypeScript's any vs. Elm's Debug.todo

TypeScript's any essentially "turns off" type checking in areas that any passes through.

In Elm:

• You can get a type that could be anything with Debug.todo, but you can't build your app with --optimize if it has Debug.todo's in it
• You will still get contradictions between inconsistent uses of a type that could be anything (see this Ellie example)

This Ellie example (with compiler error as expected) and this TypeScript playground example (with no error) show the difference.

• any can not be used in places that take never
• any vs unknown
• JSON.parse returns any, as do many core and published typings
• io-ts lets you validate JSON similar to JSON decoders in Elm
• Definitely Typed (published type definitions for NPM packages)
• Definitely Typed search
• noImplicitAny
• TypeScript `strict mode in tsconfig
• Dillon's post TypeScript's Blind Spots
• JS semantics allow types that may not be intended (like adding a string + object, '' + {} === '[object Object]')
• Function parameters are inferred to be any regardless of implementation if they aren't given an explicit type
• Type narrowing
• TypeScript has untagged unions (just called Unions) - in Elm, there are only tagged unions (called Custom Types)
• Undefined vs null
• TypeScript's Void type
• TypeScript doesn't have checked exceptions like Java (there is a discussion about this on GitHub) - Elm only has explicit errors as data that must be handled exhaustively like other data types
• Discriminated unions vs Elm custom types
• Literal types
• TypeScript allows number literal values, but arithemtic expressions return generic number values instead of literal types
• Enums
• Branded types in TypeScript vs opaque types
• Elm Radio Opaque Types episode
• Switch statements are not exhaustive - you can add an eslint rule to check that (or the never trick, assert unreachable)
• Key of operator in TypeScript
• TypeScript's type system can do some cool things that Elm can't (for better and for worse)
• Prisma
• Prisma advanced TS meetup talks
• Tuples in TypeScript are just arrays and use narrowing - Tuple in Elm is a specific type
• elm-ts-interop
• TypeScript handbook on official site