{-# LANGUAGE ApplicativeDo #-}
{-# LANGUAGE TemplateHaskell #-}

module Hasura.GraphQL.Schema.BoolExp
  ( boolExp,
    mkBoolOperator,
    equalityOperators,
    comparisonOperators,
  )
where

import Data.Text.Extended
import Hasura.GraphQL.Parser
  ( InputFieldsParser,
    Kind (..),
    Parser,
    UnpreparedValue,
  )
import Hasura.GraphQL.Parser qualified as P
import Hasura.GraphQL.Parser.Class
import Hasura.GraphQL.Parser.Constants qualified as G
import Hasura.GraphQL.Schema.Backend
import Hasura.GraphQL.Schema.Common (askTableInfo, partialSQLExpToUnpreparedValue)
import Hasura.GraphQL.Schema.Table
import Hasura.Prelude
import Hasura.RQL.Types
import Language.GraphQL.Draft.Syntax qualified as G

-- |
-- > input type_bool_exp {
-- >   _or: [type_bool_exp!]
-- >   _and: [type_bool_exp!]
-- >   _not: type_bool_exp
-- >   column: type_comparison_exp
-- >   ...
-- > }
boolExp ::
  forall b r m n.
  MonadBuildSchema b r m n =>
  SourceName ->
  TableInfo b ->
  m (Parser 'Input n (AnnBoolExp b (UnpreparedValue b)))
boolExp sourceName tableInfo = memoizeOn 'boolExp (sourceName, tableName) $ do
  tableGQLName <- getTableGQLName tableInfo
  name <- P.mkTypename $ tableGQLName <> G.__bool_exp
  let description =
        G.Description $
          "Boolean expression to filter rows from the table " <> tableName
            <<> ". All fields are combined with a logical 'AND'."

  fieldInfos <- tableSelectFields sourceName tableInfo
  tableFieldParsers <- catMaybes <$> traverse mkField fieldInfos
  recur <- boolExp sourceName tableInfo
  -- Bafflingly, ApplicativeDo doesn’t work if we inline this definition (I
  -- think the TH splices throw it off), so we have to define it separately.
  let specialFieldParsers =
        [ P.fieldOptional G.__or Nothing (BoolOr <$> P.list recur),
          P.fieldOptional G.__and Nothing (BoolAnd <$> P.list recur),
          P.fieldOptional G.__not Nothing (BoolNot <$> recur)
        ]

  pure $
    BoolAnd <$> P.object name (Just description) do
      tableFields <- map BoolFld . catMaybes <$> sequenceA tableFieldParsers
      specialFields <- catMaybes <$> sequenceA specialFieldParsers
      pure (tableFields ++ specialFields)
  where
    tableName = tableInfoName tableInfo

    mkField ::
      FieldInfo b ->
      m (Maybe (InputFieldsParser n (Maybe (AnnBoolExpFld b (UnpreparedValue b)))))
    mkField fieldInfo = runMaybeT do
      fieldName <- hoistMaybe $ fieldInfoGraphQLName fieldInfo
      P.fieldOptional fieldName Nothing <$> case fieldInfo of
        -- field_name: field_type_comparison_exp
        FIColumn columnInfo ->
          lift $ fmap (AVColumn columnInfo) <$> comparisonExps @b (ciType columnInfo)
        -- field_name: field_type_bool_exp
        FIRelationship relationshipInfo -> do
          remoteTableInfo <- askTableInfo sourceName $ riRTable relationshipInfo
          remotePermissions <- lift $ tableSelectPermissions remoteTableInfo
          let remoteTableFilter =
                fmap partialSQLExpToUnpreparedValue
                  <$> maybe annBoolExpTrue spiFilter remotePermissions
          remoteBoolExp <- lift $ boolExp sourceName remoteTableInfo
          pure $ fmap (AVRelationship relationshipInfo . andAnnBoolExps remoteTableFilter) remoteBoolExp
        FIComputedField ComputedFieldInfo {..} -> do
          let ComputedFieldFunction {..} = _cfiFunction
          -- For a computed field to qualify in boolean expression it shouldn't have any input arguments
          case toList _cffInputArgs of
            [] -> do
              let sessionArgPresence =
                    mkSessionArgumentPresence P.UVSession _cffSessionArgument _cffTableArgument
              fmap (AVComputedField . AnnComputedFieldBoolExp _cfiXComputedFieldInfo _cfiName _cffName sessionArgPresence)
                <$> case _cfiReturnType of
                  CFRScalar scalarType -> lift $ fmap CFBEScalar <$> comparisonExps @b (ColumnScalar scalarType)
                  CFRSetofTable table -> do
                    info <- askTableInfo sourceName table
                    lift $ fmap (CFBETable table) <$> boolExp sourceName info
            _ -> hoistMaybe Nothing

        -- Using remote relationship fields in boolean expressions is not supported.
        FIRemoteRelationship _ -> empty

{- Note [Nullability in comparison operators]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

In comparisonExps, we hardcode most operators with `Nullability False` when
calling `column`, which might seem a bit sketchy. Shouldn’t the nullability
depend on the nullability of the underlying Postgres column?

No. If we did that, then we would allow boolean expressions like this:

    delete_users(where: {status: {eq: null}})

which in turn would generate a SQL query along the lines of:

    DELETE FROM users WHERE users.status = NULL

but `= NULL` might not do what they expect. For instance, on Postgres, it always
evaluates to False!

Even operators for which `null` is a valid value must be careful in their
implementation. An explicit `null` must always be handled explicitly! If,
instead, an explicit null is ignored:

    foo <- fmap join $ fieldOptional "_foo_level" $ nullable int

then

       delete_users(where: {_foo_level: null})
    => delete_users(where: {})
    => delete_users()

Now we’ve gone and deleted every user in the database. Whoops! Hopefully the
user had backups!

In most cases, as mentioned above, we avoid this problem by making the column
value non-nullable (which is correct, since we never treat a null value as a SQL
NULL), then creating the field using 'fieldOptional'. This creates a parser that
rejects nulls, but won’t be called at all if the field is not specified, which
is permitted by the GraphQL specification. See Note [The value of omitted
fields] in Hasura.GraphQL.Parser.Internal.Parser for more details.

Additionally, it is worth nothing that the `column` parser *does* handle
explicit nulls, by creating a Null column value.

But... the story doesn't end there. Some of our users WANT this peculiar
behaviour. For instance, they want to be able to express the following:

    query($isVerified: Boolean) {
      users(where: {_isVerified: {_eq: $isVerified}}) {
        name
      }
    }

    $isVerified is True  -> return users who are verified
    $isVerified is False -> return users who aren't
    $isVerified is null  -> return all users

In the future, we will likely introduce a separate group of operators that do
implement this particular behaviour explicitly; but for now we have an option that
reverts to the previous behaviour.

To do so, we have to treat explicit nulls as implicit one: this is what the
'nullable' combinator does: it treats an explicit null as if the field has never
been called at all.
-}

-- This is temporary, and should be removed as soon as possible.
mkBoolOperator ::
  (MonadParse n, 'Input P.<: k) =>
  -- | shall this be collapsed to True when null is given?
  Bool ->
  -- | name of this operator
  G.Name ->
  -- | optional description
  Maybe G.Description ->
  -- | parser for the underlying value
  Parser k n a ->
  InputFieldsParser n (Maybe a)
mkBoolOperator True name desc = fmap join . P.fieldOptional name desc . P.nullable
mkBoolOperator False name desc = P.fieldOptional name desc

equalityOperators ::
  (MonadParse n, 'Input P.<: k) =>
  -- | shall this be collapsed to True when null is given?
  Bool ->
  -- | parser for one column value
  Parser k n (UnpreparedValue b) ->
  -- | parser for a list of column values
  Parser k n (UnpreparedValue b) ->
  [InputFieldsParser n (Maybe (OpExpG b (UnpreparedValue b)))]
equalityOperators collapseIfNull valueParser valueListParser =
  [ mkBoolOperator collapseIfNull G.__is_null Nothing $ bool ANISNOTNULL ANISNULL <$> P.boolean,
    mkBoolOperator collapseIfNull G.__eq Nothing $ AEQ True <$> valueParser,
    mkBoolOperator collapseIfNull G.__neq Nothing $ ANE True <$> valueParser,
    mkBoolOperator collapseIfNull G.__in Nothing $ AIN <$> valueListParser,
    mkBoolOperator collapseIfNull G.__nin Nothing $ ANIN <$> valueListParser
  ]

comparisonOperators ::
  (MonadParse n, 'Input P.<: k) =>
  -- | shall this be collapsed to True when null is given?
  Bool ->
  -- | parser for one column value
  Parser k n (UnpreparedValue b) ->
  [InputFieldsParser n (Maybe (OpExpG b (UnpreparedValue b)))]
comparisonOperators collapseIfNull valueParser =
  [ mkBoolOperator collapseIfNull G.__gt Nothing $ AGT <$> valueParser,
    mkBoolOperator collapseIfNull G.__lt Nothing $ ALT <$> valueParser,
    mkBoolOperator collapseIfNull G.__gte Nothing $ AGTE <$> valueParser,
    mkBoolOperator collapseIfNull G.__lte Nothing $ ALTE <$> valueParser
  ]