The Type System

This is a brief overview of Monte’s type system.

Monte does not have a type system, in the type-theoretic sense. Instead, Monte features Guards and Data. However, we cannot deny that guards both syntactically and semantically resemble types, so we are happy to call our guard system our “type system” and compare it to other type systems.

We use the Smallshire classification of type system features to explain Monte’s typing features in a high-level overview.


A language is untyped if there is only one type of value in the language. There are two common definitions here; one is used by Smallshire, and one is used by Harper. Both are worth considering, since Monte straddles the edge.

Smallshire gives Ruby as an example of a typed language. Ruby is a close relative of Monte, and by Smallshire’s definition, Monte is also a typed language, in this view, because objects still have innate distinct behaviors.

In constrast, Harper equates untyped and unityped languages. This would mark Ruby, and Monte too, as untyped.

We say that Monte is untyped, for reasons similar to Harper’s. Monte has a uniform calling interface, which means that any message can be sent to any object, and rejection is always done inside the object’s message-receiving code at runtime.


Monte is dynamic; it is possible to have a name for a value without restrictions on the type of the value.


Monte values have strong types which resist coercion. Indeed, in Monte, coercion is a reified object protocol. Objects do not have to be coercible, and most builtin objects cannot be coerced.


A language has nominal typing if types are identifiable, comparable, substitutable, etc. only if they are identical. Monte guards and interfaces have this property; in particular, Monte interfaces are not equal just by having the same declared names and methods.


Monte guards are manifest type annotations, which means that they are never inferred by canonical expansion.


Guards are optional and do not have to be specified. Indeed, Monte boasts gradual typing, which means that a Monte program can have any mix of guarded and unguarded names without affecting the correctness of guards.