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GHC 9.10.3 · lts/ghc-9.10.x · c74966e · 2026-09-27

Moduledhall-1.42.3Haskell2010

Dhall.Import

Dhall lets you import external expressions located either in local files or hosted on network endpoints.

To import a local file as an expression, just insert the path to the file, prepending a ./ if the path is relative to the current directory. For example, if you create a file named id with the following contents:

$ cat id
λ(a : Type) → λ(x : a) → x

Then you can use the file directly within a dhall program just by referencing the file's path:

$ dhall
./id Bool True
<Ctrl-D>
Bool

True

Imported expressions may contain imports of their own, too, which will continue to be resolved. However, Dhall will prevent cyclic imports. For example, if you had these two files:

$ cat foo
./bar
$ cat bar
./foo

... Dhall would throw the following exception if you tried to import foo:

$ dhall
./foo
^D
↳ ./foo
  ↳ ./bar

Cyclic import: ./foo

You can also import expressions hosted on network endpoints. Just use the URL

http://host[:port]/path

The compiler expects the downloaded expressions to be in the same format as local files, specifically UTF8-encoded source code text.

For example, if our id expression were hosted at http://example.com/id, then we would embed the expression within our code using:

http://example.com/id

You can also import expressions stored within environment variables using env:NAME, where NAME is the name of the environment variable. For example:

$ export FOO=1
$ export BAR='"Hi"'
$ export BAZ='λ(x : Bool) → x == False'
$ dhall <<< "{ foo = env:FOO , bar = env:BAR , baz = env:BAZ }"
{ bar : Text, baz : ∀(x : Bool) → Bool, foo : Integer }

{ bar = "Hi", baz = λ(x : Bool) → x == False, foo = 1 }

If you wish to import the raw contents of an impoert as Text then add as Text to the end of the import:

$ dhall <<< "http://example.com as Text"
Text

"<!doctype html>\n<html>\n<head>\n    <title>Example Domain</title>\n\n    <meta
 charset=\"utf-8\" />\n    <meta http-equiv=\"Content-type\" content=\"text/html
; charset=utf-8\" />\n    <meta name=\"viewport\" content=\"width=device-width,
initial-scale=1\" />\n    <style type=\"text/css\">\n    body {\n        backgro
und-color: #f0f0f2;\n        margin: 0;\n        padding: 0;\n        font-famil
y: \"Open Sans\", \"Helvetica Neue\", Helvetica, Arial, sans-serif;\n        \n
   }\n    div {\n        width: 600px;\n        margin: 5em auto;\n        paddi
ng: 50px;\n        background-color: #fff;\n        border-radius: 1em;\n    }\n
    a:link, a:visited {\n        color: #38488f;\n        text-decoration: none;
\n    }\n    @media (max-width: 700px) {\n        body {\n            background
-color: #fff;\n        }\n        div {\n            width: auto;\n            m
argin: 0 auto;\n            border-radius: 0;\n            padding: 1em;\n
  }\n    }\n    </style>    \n</head>\n\n<body>\n<div>\n    <h1>Example Domain</
h1>\n    <p>This domain is established to be used for illustrative examples in d
ocuments. You may use this\n    domain in examples without prior coordination or
 asking for permission.</p>\n    <p><a href=\"http://www.iana.org/domains/exampl
e\">More information...</a></p>\n</div>\n</body>\n</html>\n"
  • 17 types
  • 31 values
  • Packagedhall-1.42.3
  • Exports48
  • LanguageHaskell2010
  • LicenceBSD-3-Clause
  • SourceImport.hs

Import

48 declarations
valuehashExpressionToCode :: Expr Void Void -> Text
#

Convenience utility to hash a fully resolved expression and return the base-16 encoded hash with the sha256: prefix

In other words, the output of this function can be pasted into Dhall source code to add an integrity check to an import

datadata Status
#

State threaded throughout the import process

Constructors

newtypenewtype Chained
#

A fully "chained" import, i.e. if it contains a relative path that path is relative to the current directory. If it is a remote import with headers those are well-typed (either of type `List { header : Text, value Text}` or `List { mapKey : Text, mapValue Text})` and in normal form. These invariants are preserved by the API exposed by Dhall.Import.

Instances3Eq, Ord, Pretty
  • Eq ChainedDefined in dhall-1.42.3 · Dhall.Import.Types
  • Ord ChainedDefined in dhall-1.42.3 · Dhall.Import.Types
  • Pretty ChainedDefined in dhall-1.42.3 · Dhall.Import.Types
valueremoteStatus
  1. :: URL

    Public address of the server

  2. -> Status
#

Default Status appropriate for a server interpreting Dhall code

Using this Status ensures that interpreted Dhall code cannot access server-local resources (like files or environment variables)

valuetoHeaders :: Expr s a -> [HTTPHeader]
#

Given a well-typed (of type `List { header : Text, value Text }` or `List { mapKey : Text, mapValue Text }`) headers expressions in normal form construct the corresponding binary http headers; otherwise return the empty list.

valuedependencyToFile :: Status -> Import -> IO (Maybe FilePath)
#

This function is used by the --transitive option of the dhall {freeze,format,lint} subcommands to determine which dependencies to descend into

Example1 expression
dependencyToFile (emptyStatus ".") Import{ importHashed = ImportHashed{ hash = Nothing, importType = Local Here (File (Directory []) "foo") }, importMode = Code }Just "./foo"
Example1 expression
dependencyToFile (emptyStatus "./foo") Import{ importHashed = ImportHashed{ hash = Nothing, importType = Local Here (File (Directory []) "bar") }, importMode = Code }Just "./foo/bar"
Example1 expression
dependencyToFile (emptyStatus "./foo") Import{ importHashed = ImportHashed{ hash = Nothing, importType = Remote (URL HTTPS "example.com" (File (Directory []) "") Nothing Nothing) }, importMode = Code }Nothing
Example1 expression
dependencyToFile (emptyStatus ".") Import{ importHashed = ImportHashed{ hash = Nothing, importType = Env "foo" }, importMode = Code }Nothing
newtypenewtype Cycle
#

An import failed because of a cycle in the import graph

Constructors

Instances2Show, Exception
newtypenewtype ReferentiallyOpaque
#

Dhall tries to ensure that all expressions hosted on network endpoints are weakly referentially transparent, meaning roughly that any two clients will compile the exact same result given the same URL.

To be precise, a strong interpretaton of referential transparency means that if you compiled a URL you could replace the expression hosted at that URL with the compiled result. Let's call this "static linking". Dhall (very intentionally) does not satisfy this stronger interpretation of referential transparency since "statically linking" an expression (i.e. permanently resolving all imports) means that the expression will no longer update if its dependencies change.

In general, either interpretation of referential transparency is not enforceable in a networked context since one can easily violate referential transparency with a custom DNS, but Dhall can still try to guard against common unintentional violations. To do this, Dhall enforces that a non-local import may not reference a local import.

Local imports are defined as:

  • A file

  • A URL with a host of localhost or 127.0.0.1

All other imports are defined to be non-local

Constructors

Instances2Show, Exception