termlevel

Term level, often referred to as the value level, denotes the portion of a programming language where ordinary expressions are evaluated to concrete runtime values. This is in contrast to the type level, where computations concern types, kinds, and compile-time guarantees rather than values.

Term-level constructs include variables, function applications, literals, and control flow. Type-level constructs include types, type constructors, and type-level functions. In many languages with advanced type systems, the two levels interact: programs written at the term level can influence types through dependent types, while some type-level features can affect term-level behavior via reflection, reification, or singleton representations.

Examples of the relationship between levels appear in languages with dependent types and rich type systems. In Haskell, data kinds and GADTs create a bridge between term-level and type-level reasoning, and libraries that implement singletons provide values reflecting type-level information. In languages like TypeScript or Scala, type-level programming operates on types at compile time and is distinct from the runtime term level, yet you can connect the two through generic programming and type inference. In explicitly dependently-typed languages such as Idris or Coq, types can depend on values, making the line between term and type levels more fluid.

Design notes: separating term and type levels can improve safety and enable optimizations, but may add complexity and hinder rapid iteration. When needed, developers use bridges such as singleton types, type witnesses, or macro systems to connect level-specific code.

See also: dependent types, type-level programming, value level, singleton type, macros.