parsed_it - v0.1.1

parsed_it/json

JSON parsing and serialization for Gleam.

This module provides functions for parsing JSON strings into Gleam values and serializing Gleam values to JSON strings.

Parsing JSON

Use parse with a decoder for type-safe parsing:

import gleam/dynamic/decode
import parsed_it/json

type User {
  User(name: String, age: Int)
}

fn user_decoder() -> decode.Decoder(User) {
  use name <- decode.field("name", decode.string)
  use age <- decode.field("age", decode.int)
  decode.success(User(name:, age:))
}

pub fn example() {
  json.parse(from: "{\"name\":\"Lucy\",\"age\":30}", using: user_decoder())
}

Use parse_dynamic when you need to inspect raw JSON structure:

let result = json.parse_dynamic(from: json_string)

Building JSON

Use the constructor functions to build JSON values:

json.object([
  #("name", json.string("Lucy")),
  #("scores", json.array([json.int(95), json.int(87)])),
  #("active", json.bool(True)),
])
|> json.to_string

Error Handling

Parse errors are returned as JsonDecodeError variants:

Platform Notes

Erlang: Uses OTP’s built-in json module.

JavaScript: Uses native JSON.parse/JSON.stringify. Error details vary by JavaScript engine; some engines may return empty error information.

Types

Json

</>

An opaque type representing a JSON value.

This type is used for building JSON structures that can be serialized to strings. Use the constructor functions like string, int, bool, array, and object to create JSON values.

pub type Json

JsonDecodeError

</>

Errors that can occur when parsing JSON.

Note: This is a public ADT, so adding new variants would be a breaking change. Pattern matching on these variants is safe and exhaustive.

The UnexpectedChar variant contains a hex representation of the byte (e.g., “0x7B” for ‘{’) when it can be determined from the error.

pub type JsonDecodeError {
  UnexpectedEnd
  UnexpectedChar(String)
  UnexpectedSequence(String)
  UnableToDecode(List(decode.DecodeError))
}

Constructors 

  • UnexpectedEnd

    The JSON input ended unexpectedly before a complete value was parsed. This typically indicates truncated input or unclosed brackets/quotes.

  • UnexpectedChar(String)

    An unexpected character was encountered during parsing. The string contains the hex code of the character (e.g., “0x7B”). May be empty if the JavaScript engine doesn’t provide error details.

  • UnexpectedSequence(String)

    An unexpected sequence of characters was encountered during parsing. For example, an unquoted identifier where a string was expected.

  • UnableToDecode(List(decode.DecodeError))

    The JSON was valid but could not be decoded into the expected type. Contains the list of decode errors explaining what went wrong.

Values

array

</> 
pub fn array(entries: List(Json)) -> Json

Creates a JSON array from a list of JSON values.

bool

</> 
pub fn bool(input: Bool) -> Json

Creates a JSON boolean value.

float

</> 
pub fn float(input: Float) -> Json

Creates a JSON number from a float.

int

</> 
pub fn int(input: Int) -> Json

Creates a JSON number from an integer.

null

</> 
pub fn null() -> Json

Creates a JSON null value.

object

</> 
pub fn object(entries: List(#(String, Json))) -> Json

Creates a JSON object from a list of key-value pairs.

parse

</> 
pub fn parse(
  from json: String,
  using decoder: decode.Decoder(t),
) -> Result(t, JsonDecodeError)

Parses a JSON string and decodes it using the provided decoder.

Returns an error if the JSON is invalid or if the decoded value doesn’t match the expected type.

parse_dynamic

</> 
pub fn parse_dynamic(
  from json: String,
) -> Result(dynamic.Dynamic, JsonDecodeError)

Parses a JSON string into a Dynamic value without decoding.

This is useful when you want to inspect the raw JSON structure before deciding how to decode it, or when building generic JSON processing tools.

Example

let result = json.parse_dynamic(from: "{\"key\": \"value\"}")
// result contains the raw dynamic structure

string

</> 
pub fn string(input: String) -> Json

Creates a JSON string value.

to_string

</> 
pub fn to_string(json: Json) -> String

Converts a JSON value to its string representation.

to_string_tree

</> 
pub fn to_string_tree(json: Json) -> string_tree.StringTree

Converts a JSON value to a StringTree representation.

This is useful for efficiently building larger strings or for streaming output without intermediate string allocations.

Example

let tree = json.object([#("key", json.string("value"))])
  |> json.to_string_tree

Note: Currently this is implemented as to_string followed by conversion to StringTree. A future version may provide a more efficient streaming implementation.

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