advent/src/Parser/JSON.md

# JSON Parser

```idris
module Parser.JSON

import public Parser
import public Parser.Numbers

import Structures.Dependent.DList
```

<!-- idris
import System
import Derive.Prelude
import Generics.Derive

%hide Generics.Derive.Eq
%hide Generics.Derive.Ord
%hide Generics.Derive.Show

%language ElabReflection
-->

## JSON components

Types a JSON value is allowed to have

```idris
public export
data JSONType : Type where
  TObject : JSONType
  TArray : JSONType
  TString : JSONType
  TNumber : JSONType
  TBool : JSONType
  TNull : JSONType
%runElab derive "JSONType" [Generic, Meta, Eq, Ord, Show, DecEq]
%name JSONType type, type2, type3
```

A JSON value indexed by its type

```idris
public export
data JSONValue : JSONType -> Type where
  VObject : {types : List JSONType} 
    -> DList JSONType (\t => (String, JSONValue t)) types -> JSONValue TObject
  VArray : {types : List JSONType}
    -> DList JSONType JSONValue types -> JSONValue TArray
  VString : (s : String) -> JSONValue TString
  VNumber : (d : Double) -> JSONValue TNumber
  VBool : (b : Bool) -> JSONValue TBool
  VNull : JSONValue TNull
%name JSONValue value, value2, value3
```

<!-- idris
Show (JSONValue t) where
  show (VObject xs) = 
    let xs = dMap (\_,(key, value) => "\"\{key}\":\{show value}") xs
    in assert_total $ "{\{joinBy "," xs}}"
  show (VArray xs) = 
    let xs = dMap (\_,e => show e) xs
    in assert_total $ "[\{joinBy "," . map show $ xs}]"
  show (VString s) = "\"\{s}\""
  show (VNumber d) = show d
  show (VBool False) = "false"
  show (VBool True) = "true"
  show VNull = "null"

%hide Language.Reflection.TT.WithFC.value
-->

## Parsers

We are going to get mutually recursive here. Instead of using a `mutual` block,
we will use the more modern style of declaring all our types ahead of our
definitions.

```idris
export
object : Parser (JSONValue TObject)
export
array : Parser (JSONValue TArray)
export
string : Parser (JSONValue TString)
export
number : Parser (JSONValue TNumber)
export
bool : Parser (JSONValue TBool)
export
null : Parser (JSONValue TNull)
```

Define a `whitespace` character class based on the json specifications

```idris
whitespace : Parser Char
whitespace = do
  result <- 
    parseChar (\x => any (== x) $ the (List _) [' ', '\n', '\r', '\t']) id
  case result of
    GotChar char => pure char
    GotError err => throwParseError "Expected whitespace, got: \{show err}"
    EndOfInput => throwParseError "End of Input"
```

Convenience function

```idris
dpairize : {t : JSONType} -> 
  Parser (JSONValue t) -> Parser (t' : JSONType ** JSONValue t')
dpairize x = do
  x <- x
  pure (_ ** x)
```

Top level json value parser

```idris
export 
value : Parser (t : JSONType ** JSONValue t)
value = do 
  surround whitespace $ oneOfE 
    (throwParseError "Expected JSON Value") 
    (the (List _) 
      [
        dpairize object
      , dpairize array
      , dpairize string
      , dpairize number
      , dpairize bool
      , dpairize null
      ])
```

Now go through our json value types

```idris
object = do
  oneOfE 
    (throwParseError "Expected Object") 
    (the (List _) [emptyObject, occupiedObject])
  where
    emptyObject : Parser (JSONValue TObject)
    emptyObject = do
      delimited '{' '}' (nom whitespace)
      pure $ VObject Nil
    keyValue : Parser (t : JSONType ** (String, JSONValue t))
    keyValue = do
      VString key <- surround whitespace string
      _ <- parseExactChar ':'
      (typ ** val) <- value
      pure (typ ** (key, val))
    restKeyValue : Parser (t : JSONType ** (String, JSONValue t))
    restKeyValue = do
      _ <- parseExactChar ','
      keyValue
    pairs : Parser (List1 (t : JSONType ** (String, JSONValue t)))
    pairs = do
      first <- keyValue
      rest <- many restKeyValue
      pure $ first ::: rest
    occupiedObject : Parser (JSONValue TObject)
    occupiedObject = do
      val <- delimited '{' '}' pairs
      let (types ** xs) = DList.fromList (forget val)
      pure $ VObject xs
```

```idris
array = do
  oneOfE 
    (throwParseError "Expected Array") 
    (the (List _) [emptyArray, occupiedArray])
  where
    emptyArray : Parser (JSONValue TArray)
    emptyArray = do
      _ <- parseExactChar '['
      _ <- many whitespace
      _ <- parseExactChar ']'
      pure $ VArray Nil
    restValue : Parser (t : JSONType ** JSONValue t)
    restValue = do
      _ <- parseExactChar ','
      value
    occupiedArray : Parser (JSONValue TArray)
    occupiedArray = do
      _ <- parseExactChar '['
      first <- value
      rest <- many restValue
      _ <- parseExactChar ']'
      let (types ** xs) = DList.fromList (first :: rest)
      pure $ VArray xs
```

```idris
string = do
  _ <- parseExactChar '"'
  -- TODO: Handle control characters properly
  e1 <- parseError "Expected non-quote, got quote"
  e2 <- parseError "End of input"
  contents <- parseString . many $ parseCharE (not . (== '"')) (\_ => e1) e2
  _ <- parseExactChar '"'
  pure $ VString contents
```

```idris
number = do
  d <- double
  pure $ VNumber d
```

```idris
bool = do
  oneOfE
    (throwParseError "Expected Bool")
    (the (List _) [true, false])
  where
    true : Parser (JSONValue TBool)
    true = do
      _ <- exactString "true" 
      pure $ VBool True
    false : Parser (JSONValue TBool)
    false = do
      _ <- exactString "false" 
      pure $ VBool False
```

```idris
null = do
  _ <- exactString "null"
  pure VNull
```

## Unit tests

Quick smoke test

```idris
-- @@test JSON Quick Smoke
quickSmoke : IO Bool
quickSmoke = do
  let input = "{\"string\":\"string\",\"number\":5,\"true\":true,\"false\":false,\"null\":null,\"array\":[1,2,3]}"
  putStrLn input
  Right parsed <- runFirstIO object input
    | Left err => do
      printLn err
      pure False
  putStrLn "Input: \{input}\nOutput: \{show parsed}"
  case parsed of
    VObject xs => ?quickSmoke_rhs_0
    _ => pure False
```