evaluation.md

layout	title	category	tags	order
developer-doc	Evaluation and Typing	types	types evaluation	9

Evaluation and Typing

Enso is a language that has strict semantics by default, but it can still be very useful to be able to opt-in to suspended computations (thunks) for the design of certain APIs.

To that end, Enso provides a mechanism for this through the type system.

[!WARNING] Enso is using ~ and ... to defer computations and perform them lazily when needed.

The standard library defines a Suspend a type which, when used in explicit type signatures, will cause the corresponding expression to be suspended.

• The explicit calls to Suspend and force are inserted automatically by the compiler doing demand analysis.
• This demand analysis process will also ensure that there are not polynomial chains of suspend and force being inserted to ensure performance.

The actionables for this section are as follows:

• Specify this much better.

A function argument can be prefixed by ~. That instructs the Enso runtime system to provide such an arguments a thunk - a function to be evaluated later. Such a thunk function is they evaluated when the value of the argument is accessed/used. Such an evaluation is performed every time the argument value is used.

An atom argument can be prefixed by ~. That instructs the Enso runtime system to defer evaluation of the argument until it is first accessed. Then its thunk is evaluated and when the evaluation is over, the atom argument value is replaced by the computed value. Subsequent access to the atom argument will then use the already evaluated value.

• Specifying Suspension in the Type System

Specifying Suspension in the Type System

[!WARNING] The actionables for this section are:

• Actually specify how the type system interacts with eager and lazy evaluation.

Just use ~ to mark lazily computed function or atom arguments. The rest is handled by the Enso runtime system.