elm-bridge-0.4.2: Derive Elm types and Json code from Haskell types

Safe HaskellNone




This module should be used to derive the Elm instance alongside the JSON ones. The prefered usage is to convert statements such as :

$(deriveJSON defaultOptions{fieldLabelModifier = drop 4, constructorTagModifier = map toLower} ''D)


$(deriveBoth defaultOptions{fieldLabelModifier = drop 4, constructorTagModifier = map toLower} ''D)

Which will derive both the aeson and elm-bridge instances at the same time.



data Options :: * #

Options that specify how to encode/decode your datatype to/from JSON.

Options can be set using record syntax on defaultOptions with the fields below.


data SumEncoding :: * #

Specifies how to encode constructors of a sum datatype.



A constructor will be encoded to an object with a field tagFieldName which specifies the constructor tag (modified by the constructorTagModifier). If the constructor is a record the encoded record fields will be unpacked into this object. So make sure that your record doesn't have a field with the same label as the tagFieldName. Otherwise the tag gets overwritten by the encoded value of that field! If the constructor is not a record the encoded constructor contents will be stored under the contentsFieldName field.


Constructor names won't be encoded. Instead only the contents of the constructor will be encoded as if the type had a single constructor. JSON encodings have to be disjoint for decoding to work properly.

When decoding, constructors are tried in the order of definition. If some encodings overlap, the first one defined will succeed.

Note: Nullary constructors are encoded as strings (using constructorTagModifier). Having a nullary constructor alongside a single field constructor that encodes to a string leads to ambiguity.

Note: Only the last error is kept when decoding, so in the case of malformed JSON, only an error for the last constructor will be reported.


A constructor will be encoded to an object with a single field named after the constructor tag (modified by the constructorTagModifier) which maps to the encoded contents of the constructor.


A constructor will be encoded to a 2-element array where the first element is the tag of the constructor (modified by the constructorTagModifier) and the second element the encoded contents of the constructor.

defaultOptions :: Options Source #

Note that This default set of options is distinct from that in the aeson package.

defaultOptionsDropLower :: Int -> Options Source #

This generates a default set of options. The parameter represents the number of characters that must be dropped from the Haskell field names. The first letter of the field is then converted to lowercase, ie:

data Foo = Foo { _fooBarQux :: Int }
$(deriveBoth (defaultOptionsDropLower 4) ''Foo)

Will be encoded as:


Template haskell functions

deriveElmDef :: Options -> Name -> Q [Dec] Source #

Just derive the elm-bridge definitions for generating the serialization/deserialization code. It must be kept synchronized with the Haskell code manually.

deriveBoth :: Options -> Name -> Q [Dec] Source #

Equivalent to running both deriveJSON and deriveElmDef with the same options, so as to ensure the code on the Haskell and Elm size is synchronized.