class ValError(varName: String) : Exception("$varName is val, cannot reassign.")

ValError: foo is val, cannot reassign.
    at com.github.me.dynamik.interpreter.Environment.get(Environment.kt:62)
    at com.github.me.dynamik.interpreter.TreeWalker.visitVariableExpr(TreeWalker.kt:154)
    at com.github.me.dynamik.expressions.VariableExpr.evaluateBy(Expression.kt:57)

ValError: foo is val, cannot reassign.