The type environment of a program can be modeled as a graph. In the graph, a node can represent a code element including but not limited to a function, a class, an object, a variable, an expression, a script, a global, a primitive, a module, an interface, an enumerated list, an array, an alias for a type, a parameter, a property, a type, a method, a function expression, a call signature, an index signature, an object type, or a function type. An edge in the graph can represent a relationship between code elements. When the type of a code element changes, the graph can be changed to model the revised source code. Computations concerning effects of a type change are delayed until information concerning the affected code element is requested.