Dhall.JSON

This library only exports a single dhallToJSON function for translating a Dhall syntax tree to a JSON syntax tree (i.e. a Value) for the aeson library

NOTE: The yaml library uses the same Value type to represent YAML files, so you can use this to convert Dhall expressions to YAML, too

See the dhall package if you would like to transform Dhall source code into a Dhall syntax tree. Similarly, see the aeson package if you would like to translate a JSON syntax tree into JSON.

This package also provides dhall-to-json and dhall-to-yaml executables which you can use to compile Dhall source code directly to JSON or YAML for your convenience

Not all Dhall expressions can be converted to JSON since JSON is not a programming language. The only things you can convert are:

• Bools
• Naturals
• Integers
• Doubles

• Text

• Lists
• Optional values
• unions
• records

Dhall Bools translate to JSON bools:

$ dhall-to-json <<< 'True'
true
$ dhall-to-json <<< 'False'
false

Dhall numbers translate to JSON numbers:

$ dhall-to-json <<< '+2'
2
$ dhall-to-json <<< '2'
2
$ dhall-to-json <<< '2.3'
2.3

Dhall Text translates to JSON text:

$ dhall-to-json <<< '"ABC"'
"ABC"

Dhall Lists translate to JSON lists:

$ dhall-to-json <<< '[1, 2, 3] : List Integer'
[1,2,3]

Dhall Optional values translate to null if absent and the unwrapped value otherwise:

$ dhall-to-json <<< '[] : Optional Integer'
null
$ dhall-to-json <<< '[1] : Optional Integer'
1

Dhall records translate to JSON records:

$ dhall-to-json <<< '{ foo = 1, bar = True }'
{"foo":1,"bar":true}

Dhall unions translate to the wrapped value:

$ dhall-to-json <<< "< Left = +2 | Right : Natural>"
2
$ cat config
[ < Person = { age = +47, name = "John" }
  | Place  : { location : Text }
  >
, < Place  = { location = "North Pole" }
  | Person : { age : Natural, name : Text }
  >
, < Place  = { location = "Sahara Desert" }
  | Person : { age : Natural, name : Text }
  >
, < Person = { age = +35, name = "Alice" }
  | Place  : { location : Text }
  >
]
$ dhall-to-json <<< "./config"
[{"age":47,"name":"John"},{"location":"North Pole"},{"location":"Sahara Desert"},{"age":35,"name":"Alice"}]

You can preserve the name of the alternative if you wrap the value in a record with three fields:

• contents: The union literal that you want to preserve the tag of
• field: the name of the field that will store the name of the alternative
• nesting: A value of type < Inline : {} | Nested : Text >.

If nesting is set to Inline and the union literal stored in contents contains a record then the name of the alternative is stored inline within the same record. For example, this code:

    let Example = < Left : { foo : Natural } | Right : { bar : Bool } >

in  let example = constructors Example

in  let Nesting = < Inline : {} | Nested : Text >

in  let nesting = constructors Nesting

in  { field    = "name"
    , nesting  = nesting.Inline {=}
    , contents = example.Left { foo = 2 }
    }

... produces this JSON:

{
  "foo": 2,
  "name": "Left"
}

If nesting is set to Nested nestedField then the union is store underneath a field named nestedField. For example, this code:

    let Example = < Left : { foo : Natural } | Right : { bar : Bool } >

in  let example = constructors Example

in  let Nesting = < Inline : {} | Nested : Text >

in  let nesting = constructors Nesting

in  { field    = "name"
    , nesting  = nesting.Nested "value"
    , contents = example.Left { foo = 2 }
    }

... produces this JSON:

{
  "name": "Left",
  "value": {
    "foo": 2
  }
}

Also, all Dhall expressions are normalized before translation to JSON:

$ dhall-to-json <<< "True == False"
false

Dhall to JSON

dhallToJSON :: Expr s X -> Either CompileError Value

Convert a Dhall expression to the equivalent JSON expression

>>> :set -XOverloadedStrings
>>> :set -XOverloadedLists
>>> import Dhall.Core
>>> dhallToJSON (RecordLit [("foo", IntegerLit 1), ("bar", TextLit "ABC")])
Right (Object (fromList [("foo",Number 1.0),("bar",String "ABC")]))
>>> fmap Data.Aeson.encode it
Right "{\"foo\":1,\"bar\":\"ABC\"}"

omitNull :: Value -> Value

Omit record fields that are null

data Conversion

Specify whether or not to convert association lists of type List { mapKey: Text, mapValue : v } to records

Constructors

NoConversion

Conversion

Fields

• mapKey :: Text
• mapValue :: Text

convertToHomogeneousMaps :: Conversion -> Expr s X -> Expr s X

Convert association lists to homogeneous maps

This converts an association list of the form:

[ { mapKey = k0, mapValue = v0 }, { mapKey = k1, mapValue = v1 } ]

... to a record of the form:

{ k0 = v0, k1 = v1 }

parseConversion :: Parser Conversion

codeToValue

Arguments

:: Conversion

-> Text

Describe the input for the sake of error location.

-> Text

Input text.

-> IO Value

Convert a piece of Text carrying a Dhall inscription to an equivalent JSON Value

>>> :set -XOverloadedStrings
>>> import Dhall.Core
>>> Dhall.JSON.codeToValue "(stdin)" "{ a = 1 }"
>>> Object (fromList [("a",Number 1.0)])

Exceptions

data CompileError

This is the exception type for errors that might arise when translating Dhall to JSON

Because the majority of Dhall language features do not translate to JSON this just returns the expression that failed

Constructors

Unsupported (Expr X X)

Instances

Show CompileError

Defined in Dhall.JSON

Methods

showsPrec :: Int -> CompileError -> ShowS

show :: CompileError -> String

showList :: [CompileError] -> ShowS

Exception CompileError

Defined in Dhall.JSON

Methods

toException :: CompileError -> SomeException

fromException :: SomeException -> Maybe CompileError

displayException :: CompileError -> String