What are ex post facto laws? 6 types
The principle of prospective operation of penal laws prohibits the enactment of ex post facto laws. An ex post facto law has been defined as one —
 which makes an action done before the passing of the law and which was innocent when done criminal, and punishes such action; or
 which aggravates a crime or makes it greater than it was when committed; or
 which changes the punishment and inflicts a greater punishment than the law annexed to the crime when it was committed; or
 which alters the legal rules of evidence and receives less or different testimony than the law required at the time of the commission of the offense in order to convict the defendant.
The Supreme Court added two (2) more to the list, namely:
 that which assumes to regulate civil rights and remedies only but in effect imposes a penalty or deprivation of a right which when done was lawful; or
 that which deprives a person accused of a crime of some lawful protection to which he has become entitled, such as the protection of a former conviction or acquittal, or a proclamation of amnesty (Salvador v. Mapa, Jr., G.R. No. 135080, November 28, 2007).