The trial court abused it discretion in denying an evidentiary hearing on a claim under Brady v. Maryland (1963) 373 U.S. 83 that the prosecution failed to disclose they had struck a deal with a witness that he would not be psychiatrically examined until after he had testified. The witness was the only one who could identify defendant as the trigger man, so his credibility was critical, and material impeachment evidence of a witness is subject to the requirements of Brady.