dhall-1.23.0: 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) IO (Expr Src Import) Source #

Parse an expression from a Import containing a Dhall program

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

Save an expression to the specified Import

Currently this only works for cached imports and ignores other types of imports, but could conceivably work for uncached imports in the future

The main reason for this more general type is for symmetry with exprFromImport and to support doing more clever things in the future, like doing "the right thing" for uncached imports (i.e. exporting environment variables or creating files)

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

Resolve all imports within an expression

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

Generalized version of load

You can configure the desired behavior through the initial Status that you supply

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

Hash a fully resolved expression

hashExpressionToCode :: StandardVersion -> 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

assertNoImports :: MonadIO io => Expr Src Import -> io (Expr Src X) Source #

Assert than an expression is import-free

data Status m Source #

State threaded throughout the import process

emptyStatus :: FilePath -> Status IO Source #

Default starting Status, importing relative to the given directory.

cacher :: Functor f => LensLike' f (Status m) (Import -> Expr Src X -> StateT (Status m) m ()) Source #

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