Add cursor type for more safety.

Sources/SQLite/Core/Statement.swift

@@ -191,6 +191,14 @@ public final class Statement {
 
 }
 
+extension Statement {
+
+    func rowCursorNext() throws -> [Binding?]? {
+        return try step() ? Array(row) : nil
+    }
+
+}
+
 extension Statement : Sequence {
 
     public func makeIterator() -> Statement {

Sources/SQLite/Typed/Query.swift

@@ -890,13 +890,48 @@ public struct Delete : ExpressionType {
 
 }
 
+public struct RowCursor {
+    let statement: Statement
+    let columnNames: [String: Int]
+
+    public func next() throws -> Row? {
+        return try statement.rowCursorNext().flatMap { Row(columnNames, $0) }
+    }
+
+    public func map<T>(_ transform: (Row) throws -> T) throws -> [T] {
+        var elements = [T]()
+        while true {
+            if let row = try next() {
+                elements.append(try transform(row))
+            } else {
+                break
+            }
+        }
+
+        return elements
+    }
+}
+
 extension Connection {
 
+    public func prepareCursor(_ query: QueryType) throws -> RowCursor {
+        let expression = query.expression
+        let statement = try prepare(expression.template, expression.bindings)
+        return RowCursor(statement: statement, columnNames: try columnNamesForQuery(query))
+    }
+
     public func prepare(_ query: QueryType) throws -> AnySequence<Row> {
         let expression = query.expression
         let statement = try prepare(expression.template, expression.bindings)
 
-        let columnNames: [String: Int] = try {
+        let columnNames = try columnNamesForQuery(query)
+
+        return AnySequence {
+            AnyIterator { statement.next().map { Row(columnNames, $0) } }
+        }
+    }
+
+    private func columnNamesForQuery(_ query: QueryType) throws -> [String: Int] {
         var (columnNames, idx) = ([String: Int](), 0)
         column: for each in query.clauses.select.columns {
             var names = each.expression.template.characters.split { $0 == "." }.map(String.init)
@@ -937,11 +972,6 @@ extension Connection {
             idx += 1
         }
         return columnNames
-        }()
-
-        return AnySequence {
-            AnyIterator { statement.next().map { Row(columnNames, $0) } }
-        }
     }
 
     public func scalar<V : Value>(_ query: ScalarQuery<V>) throws -> V {
@@ -967,7 +997,7 @@ extension Connection {
     }
 
     public func pluck(_ query: QueryType) throws -> Row? {
-        return try prepare(query.limit(1, query.clauses.limit?.offset)).makeIterator().next()
+        return try prepareCursor(query.limit(1, query.clauses.limit?.offset)).next()
     }
 
     /// Runs an `Insert` query.

Tests/SQLiteTests/QueryTests.swift

@@ -334,6 +334,16 @@ class QueryIntegrationTests : SQLiteTestCase {
         }
     }
 
+    func test_prepareCursor() {
+        let names = ["a", "b", "c"]
+        try! InsertUsers(names)
+
+        let emailColumn = Expression<String>("email")
+        let emails = try! db.prepareCursor(users).map { $0[emailColumn] }
+
+        XCTAssertEqual(names.map({ "\($0)@example.com" }), emails.sorted())
+    }
+
     func test_scalar() {
         XCTAssertEqual(0, try! db.scalar(users.count))
         XCTAssertEqual(false, try! db.scalar(users.exists))