wip column expressions expr {}

Author: Jolanrensen

core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/api/ColumnsSelectionDsl.kt

@@ -41,6 +41,7 @@ import org.jetbrains.kotlinx.dataframe.impl.columns.top
 import org.jetbrains.kotlinx.dataframe.impl.columns.transform
 import org.jetbrains.kotlinx.dataframe.impl.columns.transformSingle
 import org.jetbrains.kotlinx.dataframe.impl.columns.tree.dfs
+import org.jetbrains.kotlinx.dataframe.documentation.*
 import kotlin.reflect.KProperty
 import kotlin.reflect.KType
 import kotlin.reflect.typeOf
@@ -638,6 +639,11 @@ public interface ColumnsSelectionDsl<out T> : ColumnSelectionDsl<T>, SingleColum
         toColumnAccessor().colsOf(type, filter)
 }
 
+/**
+ * @include [ColumnExpression]
+ *
+ * TODO
+ */
 public inline fun <T, reified R> ColumnsSelectionDsl<T>.expr(
     name: String = "",
     infer: Infer = Infer.Nulls,

core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/documentation/ColumnExpression.kt

@@ -0,0 +1,18 @@
+package org.jetbrains.kotlinx.dataframe.documentation
+
+import org.jetbrains.kotlinx.dataframe.api.*
+
+/**
+ * ## Column Expression
+ * In many DSLs, the lambda `expr {}` can be used to
+ * create a temporary new column by defining an expression to fill up each row.
+ *
+ * These DSLs include (but are not limited to):
+ * [The Add DSL][AddDsl.expr], [The Columns Selection DSL][ColumnsSelectionDsl.expr], and
+ * [The Create DataFrame DSL][CreateDataFrameDsl.expr].
+ *
+ * The `expr {}` call functions like a mapping statement iterating over the object it's called on.
+ *
+ * TODO examples
+ */
+internal interface ColumnExpression