A Lambda calculus type.

A Lambda calculus expression.

An error from parsing or evaluation.

Evaluates an expression and returns either an (Expr, Type) pair, or an Error.

import Lambda


-- Builtin values
evaluate (Int 42) --> Ok (Int 42, IntType)
evaluate (Num 3.14) --> Ok (Num 3.14, NumType)


-- Variable: x
evaluate (Var "x") --> Err (VariableNotFound "x")


-- Abstraction: λx.x
evaluate (Abs "x" (Var "x")) --> Ok (Abs "x" (Var "x"), AbsType (Type "a") (Type "a"))


-- Application: (λx.x) 42
evaluate (App (Abs "x" (Var "x")) (Int 42)) --> Ok (Int 42, IntType)