2020-05-15 13:41:26 +03:00
|
|
|
---
|
|
|
|
|
layout: developer-doc
|
|
|
|
|
title: Evaluation Semantics in Enso
|
|
|
|
|
category: semantics
|
|
|
|
|
tags: [semantics, evaluation, suspension]
|
|
|
|
|
order: 5
|
|
|
|
|
---
|
|
|
|
|
|
|
|
|
|
# Evaluation Semantics in Enso
|
2020-07-21 15:59:40 +03:00
|
|
|
|
2020-05-15 13:41:26 +03:00
|
|
|
Enso's evaluation semantics can be succinctly described as 'strict, but with
|
|
|
|
|
optional laziness'. By default, expressions in Enso are evaluated strictly, but
|
|
|
|
|
the programmer may choose to 'suspend' computations, and instead evaluate them
|
|
|
|
|
at the point they are needed.
|
|
|
|
|
|
|
|
|
|
<!-- MarkdownTOC levels="2,3" autolink="true" -->
|
|
|
|
|
|
|
|
|
|
- [Strict Evaluation](#strict-evaluation)
|
|
|
|
|
- [Optional Suspension](#optional-suspension)
|
|
|
|
|
|
|
|
|
|
<!-- /MarkdownTOC -->
|
|
|
|
|
|
|
|
|
|
## Strict Evaluation
|
2020-07-21 15:59:40 +03:00
|
|
|
|
2020-05-15 13:41:26 +03:00
|
|
|
Though Enso shares many syntactic similarities with Haskell, the most famous
|
|
|
|
|
example of a lazily evaluated language, Enso is not lazy. Instead, Enso is a
|
|
|
|
|
language that is strict.
|
|
|
|
|
|
|
|
|
|
- Statements in Enso are evaluated as soon as they are bound to a name.
|
|
|
|
|
- This means that arguments to a function are always evaluated before the
|
|
|
|
|
function is applied.
|
|
|
|
|
- Statements are _only_ evaluated when they contain fully-applied function
|
|
|
|
|
applications. Otherwise they return curried functions.
|
|
|
|
|
|
|
|
|
|
> The actionables for this section are:
|
|
|
|
|
>
|
|
|
|
|
> - Make this far better specified.
|
|
|
|
|
|
|
|
|
|
## Optional Suspension
|
2020-07-21 15:59:40 +03:00
|
|
|
|
2020-05-15 13:41:26 +03:00
|
|
|
Laziness, however, can often be quite useful for defining certain kinds of API.
|
|
|
|
|
To that end, Enso provides support for optional laziness, more specifically
|
|
|
|
|
optional _suspension_, through the built-in `Suspended` type.
|
|
|
|
|
|
|
|
|
|
- When a type `a` is wrapped in a `Suspended`, it is turned into a thunk.
|
|
|
|
|
- A value of type `Suspended a` may be forced to execute the suspended
|
|
|
|
|
computation and thereby obtain an `a`.
|
|
|
|
|
|
|
|
|
|
This forcing can take place in two ways:
|
|
|
|
|
|
|
|
|
|
- The user calls the standard library function `force : Suspended a -> a` on the
|
|
|
|
|
value.
|
|
|
|
|
- Automatically, at a site where its evaluation is demanded. The algorithm for
|
|
|
|
|
this is simple. If a value of type `Suspended a` is provided in a location
|
|
|
|
|
that expects a value of type `a`, the compiler will insert an implicit call to
|
|
|
|
|
`force` to produce the `a`.
|
|
|
|
|
|
|
|
|
|
> The actionables for this section are:
|
|
|
|
|
>
|
|
|
|
|
> - Make this far better specified.
|
