Haffin v. Mason was a United States Supreme Court case that dealt with the issue of whether a state court had the authority to issue a writ of habeas corpus to a prisoner held in a federal prison. The case arose when a prisoner, John Haffin, was held in a federal prison in the state of Ohio. Haffin sought a writ of habeas corpus from the state court, claiming that he was being held in violation of his constitutional rights. The state court granted the writ, and the federal government appealed the decision to the Supreme Court. The Supreme Court held that the state court did not have the authority to issue a writ of habeas corpus to a prisoner held in a federal prison. The Court reasoned that the writ of habeas corpus was a federal prerogative, and that the state court did not have the authority to interfere with the federal government's power to imprison individuals. The Court also noted that the writ of habeas corpus was a fundamental right, and that the state court should not be allowed to interfere with the federal government's power to protect the rights of individuals. In conclusion, the Supreme Court held that the state court did not have the authority to issue a writ of habeas corpus to a prisoner held in a federal prison. The Court reasoned that the writ of habeas corpus was a federal prerogative, and that the state court did not have the authority to interfere with the federal government's power to imprison individuals. The Court also noted that the writ of habeas corpus was a fundamental right, and that the state court should not be allowed to interfere with the federal government's power to protect the rights of individuals.

In Haffin v. Mason, the Supreme Court was tasked with determining whether a contract between two parties that had been partially performed could be enforced by law. The majority opinion held that the contract could not be enforced because it had been partially performed and thus did not meet all of the requirements for enforceability under state law. Justice Field dissented from this decision, arguing that contracts should generally be upheld when they have been substantially performed in good faith and without fraud or mistake on either side. He argued further that if one party has already received some benefit from performance of a contract then it would be unjust to allow them to repudiate their obligations under said agreement simply because full performance has yet to take place.