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layout developer-doc
title Evaluation Semantics in Enso
category semantics
tags
semantics
evaluation
suspension
order 5

Evaluation Semantics in Enso

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.

Strict Evaluation

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

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.