Name: purescript-argonaut-core
Owner: PureScript Contrib
Description: A fast, native representation for JSON, with serialization and folding
Created: 2015-07-10 18:06:56.0
Updated: 2018-03-07 10:12:09.0
Pushed: 2018-05-24 17:15:57.0
Size: 44
Language: PureScript
GitHub Committers
User | Most Recent Commit | # Commits |
---|
Other Committers
User | Most Recent Commit | # Commits |
---|
Core part of purescript-argonaut
that contains basic types for Json
, case analysis, printer and parser.
r install purescript-argonaut-core
Module documentation is published on Pursuit.
Some of Argonaut's functions might seem a bit arcane at first, so it can help to understand the underlying design decisions which make it the way it is.
One approach for modelling JSON values would be to define an algebraic data type, like this:
Json
JNull
JString String
JNumber Number
JBoolean Boolean
JArray (Array Json)
JObject (Object Json)
And indeed, some might even say this is the obvious approach.
Because Argonaut is written with the compilation target of JavaScript in mind,
it takes a slightly different approach, which is to reuse the existing data
types which JavaScript already provides. This way, the result of JavaScript's
JSON.parse
function is already a Json
value, and no extra processing is
needed before you can start operating on it. This ought to help your program
both in terms of speed and memory churn.
Much of the design of Argonaut follows naturally from this design decision.
(Or, where do Json
values come from?)
If your program is receiving JSON data as a string, you probably want the
jsonParser
function in Data.Argonaut.Parser
, which is a very simple wrapper
around JavaScript's JSON.parse
.
Otherwise, Json
values can be introduced into your program via the FFI or via
the construction functions in Data.Argonaut.Core
. Here are some examples:
n an FFI module.
rts.someNumber = 23.6;
rts.someBoolean = false;
rts.someObject = {people: [{name: "john"}, {name: "jane"}], common_interests: []};
urescript
ign import someNumber :: Json
ign import someBoolean :: Json
ign import someObject :: Json
Generally, if a JavaScript value could be returned from a call to JSON.parse
,
it's fine to import it from the FFI as Json
. So, for example, objects,
booleans, numbers, strings, and arrays are all fine, but functions are not.
The construction functions (that is, fromX
, or jsonX
) can be used as
follows:
rt Data.Tuple (Tuple(..))
rt Foreign.Object as StrMap
rt Data.Argonaut.Core as A
Number = A.fromNumber 23.6
Boolean = A.fromBoolean false
Object = A.fromObject (StrMap.fromFoldable [
Tuple "people" (A.fromArray [
A.jsonSingletonObject "name" (A.fromString "john"),
A.jsonSingletonObject "name" (A.fromString "jane")
]),
Tuple "common_interests" A.jsonEmptyArray
])
Json
valuesWe can perform case analysis for Json
values using the caseJson
function.
This function is necessary because Json
is not an algebraic data type. If
Json
were an algebraic data type, we would not have as much need for this
function, because we could perform pattern matching with a case ... of
expression instead.
The type of caseJson
is:
Json
forall a
(Unit -> a)
(Boolean -> a)
(Number -> a)
(String -> a)
(Array Json -> a)
(Object Json -> a)
Json
a
That is, caseJson
takes six functions, which all must return values of some
particular type a
, together with one Json
value. caseJson
itself also
returns a value of the same type a
.
A use of caseJson
is very similar to a case ... of
expression, as it allows
you to handle each of the six possibilities for the Json
value you passed in. Thinking of it this way, each of the six function arguments is like one of the
case alternatives.
The function that takes Unit
as an argument is for matching null
values. As there is only one possible null
value, we use the PureScript Unit
type, as correspondingly there is only one possible Unit
value.
Just like in a case ... of
expression, the final value
that the whole expression evaluates to comes from evaluating exactly one of the
'alternatives' (functions) that you pass in. In fact, you can tell that this
is the case just by looking at the type signature of caseJson
, because of a
property called parametricity (although a deeper explanation of parametricity
is outside the scope of this tutorial).
For example, imagine we had the following values defined in JavaScript and imported via the FFI:
rts.anotherNumber = 0.0;
rts.anotherArray = [0.0, {foo: 'bar'}, false];
rts.anotherObject = {foo: 1, bar: [2,2]};
Then we can match on them in PureScript using caseJson
:
ign import anotherNumber :: Json
ign import anotherArray :: Json
ign import anotherObject :: Json
cInfo :: Json -> String
cInfo = caseJson
onst "It was null")
b -> "Got a boolean: " <>
if b then "it was true!" else "It was false.")
x -> "Got a number: " <> show x)
s -> "Got a string, which was " <> Data.String.length s <>
" characters long.")
xs -> "Got an array, which had " <> Data.Array.length xs <>
" items.")
obj -> "Got an object, which had " <> Foreign.Object.size obj <>
" items.")
urescript
cInfo anotherNumber -- => "Got a number: 0.0"
cInfo anotherArray -- => "Got an array, which had 3 items."
cInfo anotherObject -- => "Got an object, which had 2 items."
caseJson
is the fundamental function for pattern matching on Json
values;
any kind of pattern matching you might want to do can be done with caseJson
.
However, caseJson
is not always comfortable to use, so Argonaut provides a
few other simpler versions for convenience. For example, the caseJsonX
functions can be used to match on a specific type. The first argument acts as a
default value, to be used if the Json
value turned out not to be that type.
For example, we can write a function which tests whether a JSON value is the
string “lol” like this:
JsonString :: forall a. a -> (String -> a) -> Json -> a
onLol = caseJsonString false (_ == "lol")
If the Json
value is not a string, the default false
is used. Otherwise,
we test whether the string is equal to “lol”.
The toX
functions also occupy a similar role: they attempt to convert Json
values into a specific type. If the json value you provide is of the right
type, you'll get a Just
value. Otherwise, you'll get Nothing
. For example,
we could have written isJsonLol
like this, too:
ring :: Json -> Maybe String
onLol json =
se toString json of
Just str -> str == "lol"
Nothing -> false