[SPARK-51967] Use discardableResult to prevent unnecessary warnings

### What changes were proposed in this pull request?

This PR aims to use `discardableResult` to prevent unnecessary warnings.

### Why are the changes needed?

To simplify the `Spark Connect Swift` API usages.

For example, `count()` API is used to execute an INSERT query. By adding `discardableResult`, we can make it clear that the result is discadable.

```swift
discardableResult
public func count() async throws -> Int64 {
```

### Does this PR introduce _any_ user-facing change?

This is a change to the unreleased version.

### How was this patch tested?

Pass the CIs.

### Was this patch authored or co-authored using generative AI tooling?

No.

Closes #95 from dongjoon-hyun/SPARK-51967.

Authored-by: Dongjoon Hyun <[email protected]>
Signed-off-by: Dongjoon Hyun <[email protected]>

Author: dongjoon-hyun

Sources/SparkConnect/Catalog.swift

@@ -114,7 +114,7 @@ public actor Catalog: Sendable {
       catalog.catType = .setCurrentCatalog(setCurrentCatalog)
       return catalog
     })
-    _ = try await df.count()
+    try await df.count()
   }
 
   /// Returns a list of catalogs in this session.
@@ -156,7 +156,7 @@ public actor Catalog: Sendable {
       catalog.catType = .setCurrentDatabase(setCurrentDatabase)
       return catalog
     })
-    _ = try await df.count()
+    try await df.count()
   }
 
   /// Returns a list of databases available across all sessions.

Sources/SparkConnect/DataFrame.swift

@@ -136,6 +136,7 @@ public actor DataFrame: Sendable {
 
   /// Return the total number of rows.
   /// - Returns: a `Int64` value.
+  @discardableResult
   public func count() async throws -> Int64 {
     let counter = Atomic(Int64(0))
 
@@ -440,6 +441,7 @@ public actor DataFrame: Sendable {
 
   /// Persist this `DataFrame` with the given storage level.
   /// - Parameter storageLevel: A storage level to apply.
+  @discardableResult
   public func persist(storageLevel: StorageLevel = StorageLevel.MEMORY_AND_DISK) async throws
     -> DataFrame
   {
@@ -456,6 +458,7 @@ public actor DataFrame: Sendable {
   /// This will not un-persist any cached data that is built upon this `DataFrame`.
   /// - Parameter blocking: Whether to block until all blocks are deleted.
   /// - Returns: A `DataFrame`
+  @discardableResult
   public func unpersist(blocking: Bool = false) async throws -> DataFrame {
     try await withGPRC { client in
       let service = Spark_Connect_SparkConnectService.Client(wrapping: client)

Sources/SparkConnect/DataFrameReader.swift

@@ -85,10 +85,11 @@ public actor DataFrameReader: Sendable {
   /// the schema inference step, and thus speed up data loading.
   /// - Parameter schema: A DDL schema string.
   /// - Returns: A `DataFrameReader`.
+  @discardableResult
   public func schema(_ schema: String) async throws -> DataFrameReader {
     // Validate by parsing.
     do {
-      _ = try await sparkSession.client.ddlParse(schema)
+      try await sparkSession.client.ddlParse(schema)
     } catch {
       throw SparkConnectError.InvalidTypeException
     }

Sources/SparkConnect/DataFrameWriter.swift

@@ -177,7 +177,7 @@ public actor DataFrameWriter: Sendable {
     var command = Spark_Connect_Command()
     command.writeOperation = write
 
-    _ = try await df.spark.client.execute(df.spark.sessionID, command)
+    try await df.spark.client.execute(df.spark.sessionID, command)
   }
 
   /// Saves the content of the `DataFrame` in CSV format at the specified path.

Sources/SparkConnect/DataFrameWriterV2.swift

@@ -147,6 +147,6 @@ public actor DataFrameWriterV2: Sendable {
 
     var command = Spark_Connect_Command()
     command.writeOperationV2 = write
-    _ = try await df.spark.client.execute(df.spark.sessionID, command)
+    try await df.spark.client.execute(df.spark.sessionID, command)
   }
 }

Sources/SparkConnect/RuntimeConf.swift

@@ -32,14 +32,14 @@ public actor RuntimeConf {
   ///   - key: A string for the configuration key.
   ///   - value: A string for the configuration value.
   public func set(_ key: String, _ value: String) async throws {
-    _ = try await client.setConf(map: [key: value])
+    try await client.setConf(map: [key: value])
   }
 
   /// Reset a configuration.
   /// - Parameters:
   ///   - key: A string for the configuration key.
   public func unset(_ key: String) async throws {
-    _ = try await client.unsetConf(keys: [key])
+    try await client.unsetConf(keys: [key])
   }
 
   /// Get a configuration.

Sources/SparkConnect/SparkConnectClient.swift

@@ -87,6 +87,7 @@ public actor SparkConnectClient {
   /// As a test connection, this sends the server `SparkVersion` request.
   /// - Parameter sessionID: A string for the session ID.
   /// - Returns: An `AnalyzePlanResponse` instance for `SparkVersion`
+  @discardableResult
   func connect(_ sessionID: String) async throws -> AnalyzePlanResponse {
     try await withGPRC { client in
       // To prevent server-side `INVALID_HANDLE.FORMAT (SQLSTATE: HY000)` exception.
@@ -137,14 +138,15 @@ public actor SparkConnectClient {
   /// Request the server to set a map of configurations for this session.
   /// - Parameter map: A map of key-value pairs to set.
   /// - Returns: Always return true.
+  @discardableResult
   func setConf(map: [String: String]) async throws -> Bool {
     try await withGPRC { client in
       let service = SparkConnectService.Client(wrapping: client)
       var request = getConfigRequestSet(map: map)
       request.clientType = clientType
       request.userContext = userContext
       request.sessionID = self.sessionID!
-      let _ = try await service.config(request)
+      _ = try await service.config(request)
       return true
     }
   }
@@ -160,7 +162,11 @@ public actor SparkConnectClient {
     request.operation.opType = .unset(unset)
     return request
   }
-
+  
+  /// Request the server to unset keys
+  /// - Parameter keys: An array of keys
+  /// - Returns: Always return true
+  @discardableResult
   func unsetConf(keys: [String]) async throws -> Bool {
     try await withGPRC { client in
       let service = SparkConnectService.Client(wrapping: client)
@@ -509,6 +515,7 @@ public actor SparkConnectClient {
     self.result.append(response)
   }
 
+  @discardableResult
   func execute(_ sessionID: String, _ command: Command) async throws -> [ExecutePlanResponse] {
     self.result.removeAll()
     try await withGPRC { client in
@@ -555,6 +562,7 @@ public actor SparkConnectClient {
   /// Parse a DDL string to ``Spark_Connect_DataType`` instance.
   /// - Parameter ddlString: A string to parse.
   /// - Returns: A ``Spark_Connect_DataType`` instance.
+  @discardableResult
   func ddlParse(_ ddlString: String) async throws -> Spark_Connect_DataType {
     try await withGPRC { client in
       let service = SparkConnectService.Client(wrapping: client)

Sources/SparkConnect/SparkFileUtils.swift

@@ -85,7 +85,7 @@ public enum SparkFileUtils {
   static func createDirectory(root: String, namePrefix: String = "spark") -> URL {
     let tempDir = URL(fileURLWithPath: root).appendingPathComponent(
       "\(namePrefix)-\(UUID().uuidString)")
-    _ = createDirectory(at: tempDir)
+    createDirectory(at: tempDir)
     return tempDir
   }
 

Sources/SparkConnect/SparkSession.swift

@@ -201,6 +201,7 @@ public actor SparkSession {
 
     /// Remove all stored configurations.
     /// - Returns: self
+    @discardableResult
     func clear() -> Builder {
       sparkConf.removeAll()
       return self

Tests/SparkConnectTests/BuilderTests.swift

@@ -42,7 +42,7 @@ struct BuilderTests {
     // Don't try to connect
     let builder = await SparkSession.builder.remote("sc://spark:1234")
     #expect(await builder.sparkConf["spark.remote"] == "sc://spark:1234")
-    _ = await builder.clear()
+    await builder.clear()
   }
 
   @Test