Functions of constitution are
(a) Constitution serves as the document that specifies the rules and regulations according to which the country functions. It establishes a system based on the rule of law, which identifies that government will function as per set norms, rules and not according to whims and fancies of a ruler. The constitution provides basic minimum rules of governance and coordination amongst the members of the society.
(b) The Constitution specifies the political system of the country, underlines the political structure of a country. For instance, the Indian Constitution declares India to be a democratic Republic. It also specifies and divides the power between the three organs of the government, it also explains the role of Parliament as a law making body.
(c) Most importantly, the document empowers the people by giving them a set of political right and guaranteeing liberty, equality and justice in the society. In this way the constitution lays down the limits of the government. For example, the chapter on the Fundamental Rights. The Constitution also prevent tyranny of the majority by recognising the rights of the minorities.
(d) A constitution besides being a legal document is also socio - economic in nature as it identifies the aims and aspirations of the society guiding our political leadership. We may give example of directive principles of state policy are basically the instruction / directives to be kept in mind while framing policies so as to bring about just society.
(e) The constitution also gives fundamental identity to the people, it serves as the basis for both political identity by specifying norms , procedures, rights and moral identity by reflecting aims and aspirations of the society.
Hope this helps:)