make fromDatatypeValue throw

When using custom types, sometimes decoding can fail, say due to
changes in the type structure. In this case decoding would fail
and the only way to handle it is forcing a crash.

This change allows you to use `try row.get()` instead. Givng you
the chance to handle the mismatch.

Author: Elfred Pagán

Sources/SQLite/Core/Value.swift

@@ -39,7 +39,7 @@ public protocol Value: Expressible { // extensions cannot have inheritance claus
 
     static var declaredDatatype: String { get }
 
-    static func fromDatatypeValue(_ datatypeValue: Datatype) -> ValueType
+    static func fromDatatypeValue(_ datatypeValue: Datatype) throws -> ValueType
 
     var datatypeValue: Datatype { get }
 

Sources/SQLite/Helpers.swift

@@ -121,9 +121,9 @@ func transcode(_ literal: Binding?) -> String {
     }
 }
 
-// swiftlint:disable force_cast
+// swiftlint:disable force_cast force_try
 func value<A: Value>(_ binding: Binding) -> A {
-    A.fromDatatypeValue(binding as! A.Datatype) as! A
+    try! A.fromDatatypeValue(binding as! A.Datatype) as! A
 }
 
 func value<A: Value>(_ binding: Binding?) -> A {

Sources/SQLite/Typed/Query.swift

@@ -1097,7 +1097,7 @@ extension Connection {
     public func scalar<V: Value>(_ query: ScalarQuery<V?>) throws -> V.ValueType? {
         let expression = query.expression
         guard let value = try scalar(expression.template, expression.bindings) as? V.Datatype else { return nil }
-        return V.fromDatatypeValue(value)
+        return try V.fromDatatypeValue(value)
     }
 
     public func scalar<V: Value>(_ query: Select<V>) throws -> V {
@@ -1108,7 +1108,7 @@ extension Connection {
     public func scalar<V: Value>(_ query: Select<V?>) throws -> V.ValueType? {
         let expression = query.expression
         guard let value = try scalar(expression.template, expression.bindings) as? V.Datatype else { return nil }
-        return V.fromDatatypeValue(value)
+        return try V.fromDatatypeValue(value)
     }
 
     public func pluck(_ query: QueryType) throws -> Row? {
@@ -1200,9 +1200,9 @@ public struct Row {
     }
 
     public func get<V: Value>(_ column: Expression<V?>) throws -> V? {
-        func valueAtIndex(_ idx: Int) -> V? {
+        func valueAtIndex(_ idx: Int) throws -> V? {
             guard let value = values[idx] as? V.Datatype else { return nil }
-            return V.fromDatatypeValue(value) as? V
+            return try V.fromDatatypeValue(value) as? V
         }
 
         guard let idx = columnNames[column.template] else {
@@ -1224,10 +1224,10 @@ public struct Row {
                     similar: columnNames.keys.filter(similar).sorted()
                 )
             }
-            return valueAtIndex(columnNames[firstIndex].value)
+            return try valueAtIndex(columnNames[firstIndex].value)
         }
 
-        return valueAtIndex(idx)
+        return try valueAtIndex(idx)
     }
 
     public subscript<T: Value>(column: Expression<T>) -> T {

Tests/SQLiteTests/Typed/RowTests.swift

@@ -85,4 +85,34 @@ class RowTests: XCTestCase {
             }
         }
     }
+
+    public func test_get_datatype_throws() {
+        // swiftlint:disable nesting
+        struct MyType: Value {
+            enum MyError: Error {
+                case failed
+            }
+
+            public static var declaredDatatype: String {
+                Blob.declaredDatatype
+            }
+
+            public static func fromDatatypeValue(_ dataValue: Blob) throws -> Data {
+                throw MyError.failed
+            }
+
+            public var datatypeValue: Blob {
+                return Blob(bytes: [])
+            }
+        }
+
+        let row = Row(["\"foo\"": 0], [Blob(bytes: [])])
+        XCTAssertThrowsError(try row.get(Expression<MyType>("foo"))) { error in
+            if case MyType.MyError.failed = error {
+                XCTAssertTrue(true)
+            } else {
+                XCTFail("unexpected error: \(error)")
+            }
+        }
+    }
 }