dhall-1.15.0: A configuration language guaranteed to terminate

Safe HaskellNone




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


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
$ cat bar

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

$ dhall
↳ ./foo
  ↳ ./bar

Cyclic import: ./foo

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


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:


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"

"<!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"


exprFromImport :: Import -> StateT Status IO (Expr Src Import) Source #

Parse an expression from a Import containing a Dhall program

load :: Expr Src Import -> IO (Expr Src X) Source #

Resolve all imports within an expression

loadWith :: MonadCatch m => (Import -> StateT Status m (Expr Src Import)) -> Context (Expr Src X) -> Normalizer X -> Expr Src Import -> m (Expr Src X) Source #

Resolve all imports within an expression using a custom typing context and Import-resolving callback in arbitrary MonadCatch monad.

loadWithContext :: Context (Expr Src X) -> Normalizer X -> Expr Src Import -> IO (Expr Src X) Source #

Resolve all imports within an expression using a custom typing context.

load = loadWithContext Dhall.Context.empty

hashExpression :: Expr s X -> Digest SHA256 Source #

Hash a fully resolved expression

hashExpressionToCode :: Expr s X -> Text Source #

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

data Status Source #

State threaded throughout the import process




  • _stack :: [Import]

    Stack of Imports that we've imported along the way to get to the current point

  • _cache :: Map Import (Expr Src X)

    Cache of imported expressions in order to avoid importing the same expression twice with different values

  • _manager :: Maybe Dynamic

    Cache for the HTTP Manager so that we only acquire it once

emptyStatus :: Status Source #

Default starting Status

newtype Cycle Source #

An import failed because of a cycle in the import graph




Show Cycle Source # 
Instance details

Defined in Dhall.Import


showsPrec :: Int -> Cycle -> ShowS #

show :: Cycle -> String #

showList :: [Cycle] -> ShowS #

Exception Cycle Source # 
Instance details

Defined in Dhall.Import

newtype ReferentiallyOpaque Source #

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

All other imports are defined to be non-local




data Imported e Source #

Extend another exception with the current import stack




Show e => Show (Imported e) Source # 
Instance details

Defined in Dhall.Import


showsPrec :: Int -> Imported e -> ShowS #

show :: Imported e -> String #

showList :: [Imported e] -> ShowS #

Exception e => Exception (Imported e) Source # 
Instance details

Defined in Dhall.Import

data PrettyHttpException Source #

Wrapper around HttpExceptions with a prettier Show instance.

In order to keep the library API constant even when the with-http Cabal flag is disabled the pretty error message is pre-rendered and the real HttpExcepion is stored in a Dynamic

data MissingFile Source #

Exception thrown when an imported file is missing


MissingFile FilePath 

newtype MissingImports Source #

List of Exceptions we encounter while resolving Import Alternatives