Why should language design go hand in hand with type system design?

According to Robert (Bob) Harper, type system is the central organizing principle in programming language theory.

• Language features: Manifestations of type structures.

• Syntax: Governed by type-defining constructs.

• Semantics: Emerged by their interactions.

According to Shriram Krishnamurthi, retrofitting type system into an untyped language can meet several challenges:

• Untyped language can encourage patterns or constructs that are hard to be soundly typed.

• Impossible to be given a precise type: eval.