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