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1. The metadata objects weren't being duplicated when duplicating
the IR. This meant that the later passes would write metadata
multiples times into one store (reference), causing wrong
behaviour at codegen time.
72 lines
2.3 KiB
Markdown
72 lines
2.3 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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- [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
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user-defined macros, but for now it would be possible to work only with a set
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of hard-coded 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
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always have a good sense of what the most common constructs in the language
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mean, rather than allowing them to be overridden outside of the stdlib.
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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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