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83 lines
2.6 KiB
Markdown
83 lines
2.6 KiB
Markdown
---
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layout: developer-doc
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title: The Enso Macro Syntax
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category: syntax
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tags: [syntax, macro]
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order: 8
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---
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# The Enso Macro Syntax
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Enso provides a macro system that allows users to perform AST to AST
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transformations on the provided pieces of code. While many languages' macros
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provide their users with access to the compilation and type-checking phases
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(scala, for example), there are a few reasons that we don't want to:
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- The power of a dependently-typed language obviates the need for the ability to
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manipulate types at compile time.
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- Syntactic macros are far more predictable than those that can perform type
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manipulation and compute values.
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- We do not want to introduce a metaprogramming system that is too complex.
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> The actionables for this section are:
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>
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> - Fully specify the macro system.
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> - Fully specify the interactions between the parser-based macro system and the
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> runtime.
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<!-- MarkdownTOC levels="2,3" autolink="true" -->
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- [Annotations](#annotations)
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- [Annotation Naming](#annotation-naming)
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- [Automatic Deriving](#automatic-deriving)
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<!-- /MarkdownTOC -->
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## Annotations
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Much like annotations on the JVM, annotations in Enso are tags that perform a
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purely syntactic transformation on the entity to which they are applied. The
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implementation of this requires both parser changes and support for user-defined
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macros, but for now it would be possible to work only with a set of hard-coded
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annotation macros.
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Annotations can be arbitrarily nested, so a set of annotation macros become
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implicitly nested inside each other:
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```ruby
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@derive Eq Debug
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@make_magic
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type Maybe a
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use Nothing
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type Just
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```
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The above example is logically translated to:
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```ruby
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derive Eq Debug
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make_magic
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type Maybe a
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use Nothing
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type Just (value : a)
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```
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In the presence of annotations and macros, it becomes more and more important
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that we are able to reserve words such as `type` to ensure that users can always
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have a good sense of what the most common constructs in the language mean,
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rather than allowing them to be overridden outside of the stdlib.
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### Annotation Naming
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The naming of annotations follows the standard rules that Enso uses for naming
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its [identifiers](./naming.md#naming-constructs). This means that they can be in
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both referent or variable form as the annotation head is _not_ a
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[pattern context](./naming.md#pattern-contexts).
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## Automatic Deriving
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In order to make the language easier to debug, we have all types automatically
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derive an interface `DebugShow`. This interface provides a function that will
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print all the significant information about the value (e.g. locations, types,
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source information, etc).
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