letbinding

Letbinding refers to a binding created by a let construct that associates one or more identifiers with values within a local scope. The identifiers defined by a let are typically visible only inside the body of the let expression. While the exact syntax varies by language, the concept is common in functional programming and in many languages that provide local scoping constructs to improve modularity and readability.

Binding semantics vary by language. In many languages, a let binding is immutable: once a name is bound, its value cannot be reassigned within the scope. Some languages offer mutable bindings through additional syntax, enabling reassignment or in-place updates. Let bindings can be recursive in certain languages via constructs such as letrec or let ... in, allowing definitions that refer to themselves or to each other.

Evaluation strategy and polymorphism also differ. In eager (strict) languages, the bound expressions are evaluated when the let is entered. In lazy languages, evaluation is deferred until the bound values are actually used, and results may be memoized. In ML and related languages, let-generalization permits polymorphic bindings: the type variables of a value bound by let can be generalized to a polymorphic type, subject to restrictions that prevent generalization for certain kinds of expressions or side effects.

Examples illustrate the concept. Scheme’s let can bind multiple variables syntactically: (let ((x 3) (y 4)) (+ x y)). Haskell uses let x = 3 in let y = 4 in x + y. OCaml or other ML-family languages express similar bindings with in-clauses, such as let x = 3 in let y = 4 in x + y.

Overall, letbinding is a foundational construct for introducing locally scoped, often immutable names in a program, with variations in recursion, evaluation, and polymorphism across languages.