This module allows you to build decoders that elm-pages
can optimize for you in your StaticHttp
requests.
It does this by stripping of unused fields during the CLI build step. When it runs in production, it will
just run a plain elm/json
decoder, so you're fetching and decoding the stripped-down data, but without any
performance penalty.
For convenience, this library also includes a Json.Decode.Exploration.Pipeline
module which is largely a copy of NoRedInk/elm-decode-pipeline
.
Json.Decode.Error
A simple type alias for Json.Decode.Error
.
errorToString : Json.Decode.Error -> String
A simple wrapper for Json.Decode.errorToString
.
Internal.OptimizedDecoder a
A decoder that will be optimized in your production bundle.
string : Decoder String
Decode a string.
import List.Nonempty exposing (Nonempty(..))
import Json.Decode.Exploration.Located exposing (Located(..))
import Json.Encode as Encode
""" "hello world" """
|> decodeString string
--> Success "hello world"
""" 123 """
|> decodeString string
--> Errors (Nonempty (Here <| Expected TString (Encode.int 123)) [])
bool : Decoder Basics.Bool
Decode a boolean value.
""" [ true, false ] """
|> decodeString (list bool)
--> Success [ True, False ]
int : Decoder Basics.Int
Decode a number into an Int
.
import List.Nonempty exposing (Nonempty(..))
import Json.Decode.Exploration.Located exposing (Located(..))
import Json.Encode as Encode
""" 123 """
|> decodeString int
--> Success 123
""" 0.1 """
|> decodeString int
--> Errors <|
--> Nonempty
--> (Here <| Expected TInt (Encode.float 0.1))
--> []
float : Decoder Basics.Float
Decode a number into a Float
.
import List.Nonempty exposing (Nonempty(..))
import Json.Decode.Exploration.Located exposing (Located(..))
import Json.Encode as Encode
""" 12.34 """
|> decodeString float
--> Success 12.34
""" 12 """
|> decodeString float
--> Success 12
""" null """
|> decodeString float
--> Errors (Nonempty (Here <| Expected TNumber Encode.null) [])
Json.Decode.Value
A simple type alias for Json.Decode.Value
.
nullable : Decoder a -> Decoder (Maybe a)
Decodes successfully and wraps with a Just
. If the values is null
succeeds with Nothing
.
""" [ { "foo": "bar" }, { "foo": null } ] """
|> decodeString (list <| field "foo" <| nullable string)
--> Success [ Just "bar", Nothing ]
list : Decoder a -> Decoder (List a)
Decode a list of values, decoding each entry with the provided decoder.
import List.Nonempty exposing (Nonempty(..))
import Json.Decode.Exploration.Located exposing (Located(..))
import Json.Encode as Encode
""" [ "foo", "bar" ] """
|> decodeString (list string)
--> Success [ "foo", "bar" ]
""" [ "foo", null ] """
|> decodeString (list string)
--> Errors <|
--> Nonempty
--> (AtIndex 1 <|
--> Nonempty (Here <| Expected TString Encode.null) []
--> )
--> []
array : Decoder a -> Decoder (Array a)
Convenience function. Decode a JSON array into an Elm Array
.
import Array
""" [ 1, 2, 3 ] """
|> decodeString (array int)
--> Success <| Array.fromList [ 1, 2, 3 ]
dict : Decoder v -> Decoder (Dict String v)
Convenience function. Decode a JSON object into an Elm Dict String
.
import Dict
""" { "foo": "bar", "bar": "hi there" } """
|> decodeString (dict string)
--> Success <| Dict.fromList
--> [ ( "bar", "hi there" )
--> , ( "foo", "bar" )
--> ]
keyValuePairs : Decoder a -> Decoder (List ( String, a ))
Decode a JSON object into a list of key-value pairs. The decoder you provide will be used to decode the values.
""" { "foo": "bar", "hello": "world" } """
|> decodeString (keyValuePairs string)
--> Success [ ( "foo", "bar" ), ( "hello", "world" ) ]
field : String -> Decoder a -> Decoder a
Decode the content of a field using a provided decoder.
import List.Nonempty as Nonempty
import Json.Decode.Exploration.Located exposing (Located(..))
import Json.Encode as Encode
""" { "foo": "bar" } """
|> decodeString (field "foo" string)
--> Success "bar"
""" [ { "foo": "bar" }, { "foo": "baz", "hello": "world" } ] """
|> decodeString (list (field "foo" string))
--> WithWarnings expectedWarnings [ "bar", "baz" ]
expectedWarnings : Warnings
expectedWarnings =
UnusedField "hello"
|> Here
|> Nonempty.fromElement
|> AtIndex 1
|> Nonempty.fromElement
at : List String -> Decoder a -> Decoder a
Decodes a value at a certain path, using a provided decoder. Essentially,
writing at [ "a", "b", "c" ] string
is sugar over writing
field "a" (field "b" (field "c" string))
}.
""" { "a": { "b": { "c": "hi there" } } } """
|> decodeString (at [ "a", "b", "c" ] string)
--> Success "hi there"
index : Basics.Int -> Decoder a -> Decoder a
Decode a specific index using a specified Decoder
.
import List.Nonempty exposing (Nonempty(..))
import Json.Decode.Exploration.Located exposing (Located(..))
import Json.Encode as Encode
""" [ "hello", 123 ] """
|> decodeString (map2 Tuple.pair (index 0 string) (index 1 int))
--> Success ( "hello", 123 )
""" [ "hello", "there" ] """
|> decodeString (index 1 string)
--> WithWarnings (Nonempty (AtIndex 0 (Nonempty (Here (UnusedValue (Encode.string "hello"))) [])) [])
--> "there"
optionalField : String -> Decoder a -> Decoder (Maybe a)
If a field is missing, succeed with Nothing
. If it is present, decode it
as normal and wrap successes in a Just
.
When decoding with
maybe
,
if a field is present but malformed, you get a success and Nothing.
optionalField
gives you a failed decoding in that case, so you know
you received malformed data.
Examples:
import Json.Decode exposing (..)
import Json.Encode
Let's define a stuffDecoder
that extracts the "stuff"
field, if it exists.
stuffDecoder : Decoder (Maybe String)
stuffDecoder =
optionalField "stuff" string
If the "stuff" field is missing, decode to Nothing.
""" { } """
|> decodeString stuffDecoder
--> Ok Nothing
If the "stuff" field is present but not a String, fail decoding.
expectedError : Error
expectedError =
Failure "Expecting a STRING" (Json.Encode.list identity [])
|> Field "stuff"
""" { "stuff": [] } """
|> decodeString stuffDecoder
--> Err expectedError
If the "stuff" field is present and valid, decode to Just String.
""" { "stuff": "yay!" } """
|> decodeString stuffDecoder
--> Ok <| Just "yay!"
Definition from the json-extra package: https://github.com/elm-community/json-extra.
maybe : Decoder a -> Decoder (Maybe a)
Decodes successfully and wraps with a Just
, handling failure by succeeding
with Nothing
.
import List.Nonempty as Nonempty
import Json.Decode.Exploration.Located exposing (Located(..))
import Json.Encode as Encode
""" [ "foo", 12 ] """
|> decodeString (list <| maybe string)
--> WithWarnings expectedWarnings [ Just "foo", Nothing ]
expectedWarnings : Warnings
expectedWarnings =
UnusedValue (Encode.int 12)
|> Here
|> Nonempty.fromElement
|> AtIndex 1
|> Nonempty.fromElement
oneOf : List (Decoder a) -> Decoder a
Tries a bunch of decoders. The first one to not fail will be the one used.
If all fail, the errors are collected into a BadOneOf
.
import List.Nonempty as Nonempty
import Json.Decode.Exploration.Located exposing (Located(..))
import Json.Encode as Encode
""" [ 12, "whatever" ] """
|> decodeString (list <| oneOf [ map String.fromInt int, string ])
--> Success [ "12", "whatever" ]
""" null """
|> decodeString (oneOf [ string, map String.fromInt int ])
--> Errors <| Nonempty.fromElement <| Here <| BadOneOf
--> [ Nonempty.fromElement <| Here <| Expected TString Encode.null
--> , Nonempty.fromElement <| Here <| Expected TInt Encode.null
--> ]
lazy : (() -> Decoder a) -> Decoder a
Required when using (mutually) recursive decoders.
value : Decoder Value
Extract a piece without actually decoding it.
If a structure is decoded as a value
, everything in the structure will be
considered as having been used and will not appear in UnusedValue
warnings.
import Json.Encode as Encode
""" [ 123, "world" ] """
|> decodeString value
--> Success (Encode.list identity [ Encode.int 123, Encode.string "world" ])
null : a -> Decoder a
Decode a null
and succeed with some value.
""" null """
|> decodeString (null "it was null")
--> Success "it was null"
Note that undefined
and null
are not the same thing. This cannot be used to
verify that a field is missing, only that it is explicitly set to null
.
""" { "foo": null } """
|> decodeString (field "foo" (null ()))
--> Success ()
import List.Nonempty exposing (Nonempty(..))
import Json.Decode.Exploration.Located exposing (Located(..))
import Json.Encode as Encode
""" { } """
|> decodeString (field "foo" (null ()))
--> Errors <|
--> Nonempty
--> (Here <| Expected (TObjectField "foo") (Encode.object []))
--> []
succeed : a -> Decoder a
A decoder that will ignore the actual JSON and succeed with the provided value. Note that this may still fail when dealing with an invalid JSON string.
If a value in the JSON ends up being ignored because of this, this will cause a warning.
import List.Nonempty exposing (Nonempty(..))
import Json.Decode.Exploration.Located exposing (Located(..))
import Json.Encode as Encode
""" null """
|> decodeString (value |> andThen (\_ -> succeed "hello world"))
--> Success "hello world"
""" null """
|> decodeString (succeed "hello world")
--> WithWarnings
--> (Nonempty (Here <| UnusedValue Encode.null) [])
--> "hello world"
""" foo """
|> decodeString (succeed "hello world")
--> BadJson
fail : String -> Decoder a
Ignore the json and fail with a provided message.
import List.Nonempty exposing (Nonempty(..))
import Json.Decode.Exploration.Located exposing (Located(..))
import Json.Encode as Encode
""" "hello" """
|> decodeString (fail "failure")
--> Errors (Nonempty (Here <| Failure "failure" (Just <| Encode.string "hello")) [])
andThen : (a -> Decoder b) -> Decoder a -> Decoder b
Chain decoders where one decoder depends on the value of another decoder.
Note: If you run out of map functions, take a look at the pipeline module which makes it easier to handle large objects.
map : (a -> b) -> Decoder a -> Decoder b
Useful for transforming decoders.
""" "foo" """
|> decodeString (map String.toUpper string)
--> Success "FOO"
map2 : (a -> b -> c) -> Decoder a -> Decoder b -> Decoder c
Combine 2 decoders.
map3 : (a -> b -> c -> d) -> Decoder a -> Decoder b -> Decoder c -> Decoder d
Combine 3 decoders.
map4 : (a -> b -> c -> d -> e) -> Decoder a -> Decoder b -> Decoder c -> Decoder d -> Decoder e
Combine 4 decoders.
map5 : (a -> b -> c -> d -> e -> f) -> Decoder a -> Decoder b -> Decoder c -> Decoder d -> Decoder e -> Decoder f
Combine 5 decoders.
map6 : (a -> b -> c -> d -> e -> f -> g) -> Decoder a -> Decoder b -> Decoder c -> Decoder d -> Decoder e -> Decoder f -> Decoder g
Combine 6 decoders.
map7 : (a -> b -> c -> d -> e -> f -> g -> h) -> Decoder a -> Decoder b -> Decoder c -> Decoder d -> Decoder e -> Decoder f -> Decoder g -> Decoder h
Combine 7 decoders.
map8 : (a -> b -> c -> d -> e -> f -> g -> h -> i) -> Decoder a -> Decoder b -> Decoder c -> Decoder d -> Decoder e -> Decoder f -> Decoder g -> Decoder h -> Decoder i
Combine 8 decoders.
andMap : Decoder a -> Decoder (a -> b) -> Decoder b
Decode an argument and provide it to a function in a decoder.
decoder : Decoder String
decoder =
succeed (String.repeat)
|> andMap (field "count" int)
|> andMap (field "val" string)
""" { "val": "hi", "count": 3 } """
|> decodeString decoder
--> Success "hihihi"
fromResult : Result String value -> Decoder value
Turn a Result into a Decoder (uses succeed and fail under the hood). This is often
helpful for chaining with andThen
.
Usually you'll be passing your decoders to
decodeString : Decoder a -> String -> Result Error a
A simple wrapper for Json.Decode.errorToString
.
This will directly call the raw elm/json
decoder that is stored under the hood.
decodeValue : Decoder a -> Value -> Result Error a
A simple wrapper for Json.Decode.errorToString
.
This will directly call the raw elm/json
decoder that is stored under the hood.
decoder : Decoder a -> Json.Decode.Decoder a
Usually you'll want to directly pass your OptimizedDecoder
to StaticHttp
or other elm-pages
APIs.
But if you want to re-use your decoder somewhere else, it may be useful to turn it into a plain elm/json
decoder.