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120 lines
6.0 KiB
Markdown
120 lines
6.0 KiB
Markdown
Idris 2
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=======
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[](https://idris2.readthedocs.io/en/latest/?badge=latest)
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[](https://github.com/idris-lang/Idris2/actions/workflows/ci-idris2-and-libs.yml?query=branch%3Amain)
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[Idris 2](https://idris-lang.org/) is a purely functional programming language
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with first class types.
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For full installation instructions, see [INSTALL.md](INSTALL.md). Briefly, if
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you have Chez Scheme installed, with the executable name `chez`, type:
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* `make bootstrap SCHEME=chez`
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* `make install`
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You may need to change `chez` to be the local name of your Chez Scheme. This
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is often one of `scheme`, `chezscheme` or `chezscheme9.5` (depending on the
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version). On a modern desktop machine, this process (including tests)
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should take less than 5 minutes.
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Idris 2 is mostly backward compatible with Idris 1, with some minor exceptions.
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The most notable user visible differences, which might cause Idris 1 programs
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to fail to type check, are:
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+ Unbound implicit arguments are always erased, so it is a type error to
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attempt to pattern match on one.
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+ Simplified resolution of ambiguous names, which might mean you need to
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explicitly disambiguate more often. As a general rule, Idris 2 will be able
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to disambiguate between names which have different concrete return types
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(such as data constructors), or which have different concrete argument
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types (such as record projections). It may struggle to resolve ambiguities
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if one name requires an interface to be resolved.
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+ The `cong` function now takes its congruence explicitly as its first argument.
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+ Minor differences in the meaning of export modifiers `private`, `export`,
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and `public export`, which now refer to visibility of names from other
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*namespaces* rather than visibility from other *files*.
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+ Module names must match the filename in which they are defined (unless
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the module's name is "Main").
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+ Anything which uses a `%language` pragma in Idris 1 is likely to be different.
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Notably, elaborator reflection will exist, but most likely in a slightly
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different form because the internal details of the elaborator are different.
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+ The `Prelude` is much smaller (and easier to replace with an alternative).
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Command-line option `--no-prelude` can be used to not implicitly import `Prelude`.
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+ `let x = val in e` no longer computes with `x` in `e`, instead being
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essentially equivalent to `(\x => e) val`. This is to make the
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behaviour of `let` consistent in the presence of `case` and `with` (where
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it is hard to push the computation inside the `case`/`with` efficiently).
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Instead, you can define functions locally with `let`, which do have
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computational force, as follows:
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let x : ?
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x = val in
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e
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Watch this space for more details and the rationale for the changes, as I
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get around to writing it...
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Summary of new features:
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+ A core language based on "Quantitative Type Theory" which allows explicit
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annotation of erased types, and linear types.
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+ `let` bindings are now more expressive, and can be used to define pattern
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matching functions locally.
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+ Names which are in scope in a type are also always in scope in the body of
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the corresponding definition.
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+ Better inference. Holes are global to a source file, rather than local to
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a definition, meaning that some holes can be left in function types to be
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inferred by the type checker. This also gives better inference for the types
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of `case` expressions, and means fewer annotations are needed in interface
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declarations.
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+ Better type checker implementation which minimises the need for compile
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time evaluation.
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+ New Chez Scheme based backend which both compiles and runs faster than the
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default Idris 1 backend. (Also, optionally, Racket and Gambit can be used
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as targets).
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+ Everything works faster :).
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A significant change in the implementation is that there is an intermediate
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language `TTImp`, which is essentially a desugared Idris, and is cleanly
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separated from the high level language which means it is potentially usable
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as a core language for other high level syntaxes.
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JavaScript
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----------
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The JavaScript codegen uses the new BigInt, hence Node.js 10.4 or higher is required.
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Editor Plugins
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--------------
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The [wiki lists the current plugins available for common text editors](
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https://github.com/idris-lang/Idris2/wiki/Editor-Support)
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and their features.
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Things still missing
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--------------------
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+ Cumulativity (so we currently have Type : Type! Bear that in mind when you
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think you've proved something :))
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+ 'rewrite' doesn't yet work on dependent types
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Contributions wanted
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-------------------
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+ [Contribution guidelines](CONTRIBUTING.md)
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+ [Good first issues](https://github.com/idris-lang/Idris2/issues?q=is%3Aopen+is%3Aissue+label%3A%22good+first+issue%22)
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+ [Contributors wanted](https://github.com/idris-lang/Idris2/wiki/What-Contributions-are-Needed)
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If you want to learn about Idris more, contributing to the compiler could be one
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way to do so. Just select one good first issue and ask about it on the [Discord](https://discord.gg/UX68fDs2jc) channel.
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Talks
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-----
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[](https://www.youtube.com/watch?v=nbClauMCeds "Edwin Brady Tells Us What's New in Idris 2 (Berlin Functional Programming Group)")
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[](https://www.youtube.com/watch?v=h9YAOaBWuIk "Scheme Workshop Keynote (ACM SIGPLAN)")
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[](https://www.youtube.com/watch?v=DRq2NgeFcO0 "Idris 2 - Type-driven Development of Idris (Curry On - London 2019)")
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[](https://www.youtube.com/watch?v=mOtKD7ml0NU "Idris 2: Type-driven Development of Idris (Code Mesh LDN 18)")
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