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+# Dhall for Ruby
+
+This is a Ruby implementation of the Dhall configuration language. Dhall is a powerful, but safe and non-Turing-complete configuration language. For more information, see: https://dhall-lang.org
+
+## Versioning and Standard Compliance
+
+This project follows semantic versioning, and every tagged version claims to adhere to the version of the dhall-lang standard that is linked in the dhall-lang submodule.
+
+For the purposes of considering what is a "breaking change" only the API as documented in this documentation is considered, regardless of any other exposed parts of the library. Anything not documented here may change at any time, but backward-incompatible changes to anything documented here will be accompanied by a major-version increment.
+
+## Load Expressions
+
+ require "dhall"
+ Dhall.load("1 + 1").then do |value|
+ value # => #<Dhall::Natural value=2>
+ end
+
+ Dhall.load("./path/to/config.dhall").then do |config|
+ # ... use config from file
+ end
+
+ Dhall.load("https://example.com/config.dhall").then do |config|
+ # ... use config from URL
+ end
+
+`Dhall.load` will parse a Dhall expression, resolve imports, check that the types are correct, and fully normalize. The result is returned as a `Promise` to enable using import resolvers that use async I/O.
+
+### Non-Async Load
+
+Wherever possible, you should use the `Promise` API and treat `Dhall.load` as an async operation. *If* that is not possible, or *if* you know you are using a resolver that is not async, or *if* you know that there are no imports in your expression, you may use the escape hatch:
+
+ Dhall.load("1 + 1").sync # => #<Dhall::Natural value=2>
+
+**This will block the thread it is run from until the whole load operation is complete. Never call #sync from an async context.**
+
+### Customising Import Resolution
+
+You may optionally pass `Dhall.load` a resolver that will be used to resolve all imports during the load process:
+
+ Dhall.load(expr, resolver: some_resolver)
+
+There are a few provided resolvers for you to choose from:
+
+* `Dhall::Resolvers::Default` supports loading from http, https, local path, and IPFS sources. IPFS imports will come from the local moutpoint, if present, with automatic fallbacks to the local gateway, if present, and finally a public gateway.
+* `Dhall::Resolvers::Standard` should be used if you want strict dhall-lang standard compliance. It supports loading from http, https, and local paths.
+* `Dhall::Resolvers::LocalOnly` only allows imports from local paths.
+* `Dhall::Resolvers::None` will not allow any imports.
+
+It is possible to customise these options further, or provide your own resolver, but this is undocumented for now.
+
+## Function
+
+A Dhall expression may be a function, which can be used like any other Ruby proc:
+
+ Dhall.load("\\(x: Natural) -> x + 1").then do |f|
+ f.call(1) # => #<Dhall::Natural value=2>
+ f[1] # => #<Dhall::Natural value=2>
+ [1,2].map(&f) # => [#<Dhall::Natural value=2>, #<Dhall::Natural value=3>]
+ f.to_proc # => #<Proc:0xXXXX (lambda)>
+ end
+
+A curried function may be called either curried or uncurried:
+
+ Dhall.load("\\(x: Natural) -> \\(y: Natural) -> x + y").then do |f|
+ f.call(1).call(1) # => #<Dhall::Natural value=2>
+ f.call(1, 1) # => #<Dhall::Natural value=2>
+ end
+
+## Boolean
+
+A Dhall expression may be a boolean, which supports some common messages:
+
+ Dhall.load("True").then do |bool|
+ bool & false # => false
+ bool | false # => #<Dhall::Bool value=true>
+ !bool # => #<Dhall::Bool value=false>
+ bool === true # => true
+ bool.reduce(true, false) # => true
+ bool.to_s # => "True"
+ end
+
+If you need an actual instance of `TrueClass` or `FalseClass`, the suggested method is `bool === true`.
+
+## Natural
+
+A Dhall expression may be a natural (positive) number, which supports some common messages:
+
+ Dhall.load("1").then do |nat|
+ nat + 1 # => #<Dhall::Natural value=2>
+ 1 + nat # => #<Dhall::Natural value=2>
+ nat * 2 # => #<Dhall::Natural value=2>
+ 2 * nat # => #<Dhall::Natural value=2>
+ nat === 1 # => true
+ nat.zero? # => false
+ nat.even? # => false
+ nat.odd? # => true
+ nat.pred # => #<Dhall::Natural value=0>
+ nat.to_s # => "1"
+ nat.to_i # => 1
+ end
+
+## Integer
+
+A Dhall expression may be an integer (positive or negative). Dhall integers are opaque, and support fewer operations than naturals:
+
+ Dhall.load("+1").then do |int|
+ int === 1 # => true
+ int.to_s # "+1"
+ int.to_i # 1
+ end
+
+## Double
+
+A Dhall expression may be a double-precision floating point number. Dhall doubles are opaque, and support fewer operations than naturals:
+
+ Dhall.load("1.0").then do |double|
+ double === 1.0 # => true
+ double.to_s # "1.0"
+ double.to_f # 1.0
+ end
+
+## Text
+
+A Dhall expression may be a string of text, which supports some common messages:
+
+ Dhall.load("\"abc\"").then do |text|
+ text === "abc" # => true
+ text.to_s # "abc"
+ end
+
+## Optional
+
+A Dhall expression may be optionally present, like so:
+
+ Dhall.load("Some 1").then do |some|
+ some.map { |x| x + 1 } # => #<Dhall::Optional value=#<Dhall::Natural value=2> value_type=nil>
+ some.map(type: dhall_type) { ... } # => #<Dhall::Optional value=... value_type=dhall_type>
+ some.reduce(nil) { |x| x } # => #<Dhall::Natural value=1>
+ some.to_s # => 1
+ end
+
+ Dhall.load("None Natural").then do |none|
+ none.map { |x| x + 1 } # => #<Dhall::OptionalNone ...>
+ none.map(type: dhall_type) { ... } # => #<Dhall::OptionalNone value_type=dhall_type>
+ none.reduce(nil) { |x| x } # => nil
+ none.to_s # => ""
+ end
+
+## List
+
+A Dhall expression may be a list of other expressions. Lists are `Enumerable` and support all operations that entails, with some special cases:
+
+ Dhall.load("[1,2]").then do |list|
+ list.map { |x| x + 1 } # => #<Dhall::List elements=[#<Dhall::Natural value=2>, #<Dhall::Natural value=3>] element_type=nil>
+ list.map(type: dhall_type) { ... } # => #<Dhall::List elements=[...] element_type=dhall_type>
+ list.reduce(nil) { |x, _| x } # => #<Dhall::Natural value=1>
+ list.first # => #<Dhall::Optional value=#<Dhall::Natural value=1> value_type=...>
+ list.last # => #<Dhall::Optional value=#<Dhall::Natural value=2> value_type=...>
+ list[0] # => #<Dhall::Optional value=#<Dhall::Natural value=1> value_type=...>
+ list[100] # => #<Dhall::OptionalNone value_type=...>
+ list.reverse # => #<Dhall::List elements=[#<Dhall::Natural value=2>, #<Dhall::Natural value=1>] element_type=...>
+ list.join(",") # => "1,2"
+ list.to_a # => [1,2]
+ end
+
+## Record
+
+A Dhall expression may be a record of keys mapped to other expressions. Records are `Enumerable` and support many common operations:
+
+ Dhall.load("{ a = 1 }").then do |rec|
+ rec["a"] # => #<Dhall::Natural value=1>
+ rec[:a] # => #<Dhall::Natural value=1>
+ rec["b"] # => nil
+ rec.fetch("a") # => #<Dhall::Natural value=1>
+ rec.fetch(:a) # => #<Dhall::Natural value=1>
+ rec.fetch(:b) # => raise KeyError
+ rec.dig(:a) # => #<Dhall::Natural value=1>
+ rec.dig(:b) # => nil
+ rec.slice(:a) # => #<Dhall::Record a=#<Dhall::Natural value=1>>
+ rec.slice # => #<Dhall::EmptyRecord >
+ rec.keys # => ["a"]
+ rec.values # => [#<Dhall::Natural value=1>]
+ rec.map { |k, v| [k, v + 1] } # => #<Dhall::Record a=#<Dhall::Natural value=2>>
+ rec.merge(b: 2) # => #<Dhall::Record a=#<Dhall::Natural value=1> b=#<Dhall::Natural value=2>>
+ rec.deep_merge(b: 2) # => #<Dhall::Record a=#<Dhall::Natural value=1> b=#<Dhall::Natural value=2>>
+ end
+
+## Union
+
+A Dhall expression may be a union or enum. These support both a way to handle each case, and a less safe method to extract a dynamically typed object:
+
+ Dhall.load("< one | two >.one").then do |enum|
+ enum.to_s # => "one"
+ enum.reduce(one: 1, two: 2) # => 1
+ enum.extract # :one
+ end
+
+ Dhall.load("< Natural: Natural | Text: Text >.Natural 1").then do |union|
+ union.to_s # => "1"
+ union.reduce(Natural: :to_i, Text: :to_i) # => 1
+ union.extract # => #<Dhall::Natural value=1>
+ end
+
+## Serializing Expressions
+
+Dhall expressions may be serialized to a binary format for consumption by machines:
+
+ expression.to_binary
+
+If you are writing out an expression for later editing by a human, you should get [the Dhall command line tools](https://github.com/dhall-lang/dhall-haskell/releases) for your platform to make these easier to work with. You can pretty print the binary format for human editing like so:
+
+ dhall decode < path/to/binary/expression.dhallb
+
+## Semantic Hash
+
+Dhall expressions support creating a "semantic hash" that is the same for all expressions with the same normal form. This makes it very useful as a cache key or an integrity check, since formatting changes to the source code will not change the hash:
+
+ expression.cache_key
+
+## Serializing Ruby Objects
+
+You may wish to convert your existing Ruby objects to Dhall expressions. This can be done using the AsDhall refinement:
+
+ using Dhall::AsDhall
+ 1.as_dhall # => #<Dhall::Natural value=1>
+ {}.as_dhall # => #<Dhall::EmptyRecord >
+
+Many methods on Dhall expressions call `#as_dhall` on their arguments, so you can define it on your own objects to produce a custom serialization.
+
+## Porting from YAML or JSON Configuration
+
+To aid in converting your existing configurations or seralized data, there are included some experimental scripts:
+
+ json-to-dhall < path/to/config.json | dhall decode
+ yaml-to-dhall < path/to/config.yaml | dhall decode
+
+## Getting Help
+
+If you have any questions about this library, or wish to report a bug, please send email to: dev@singpolyma.net
+
+## Contributing
+
+If you have code or patches you wish to contribute, the maintainer's preferred mechanism is a git pull request. Push your changes to a git repository somewhere, for example:
+
+ git remote rename origin upstream
+ git remote add origin git@git.sr.ht:~yourname/dhall-ruby
+ git push -u origin master
+
+Then generate the pull request:
+
+ git fetch upstream master
+ git request-pull -p upstream/master origin
+
+And copy-paste the result into a plain-text email to: dev@singpolyma.net
+
+You may alternately use a patch-based approach as described on https://git-send-email.io
+
+Contributions follow an inbound=outbound model -- you (or your employer) keep all copyright on your patches, but agree to license them according to this project's COPYING file.