In yā ayyuhā al-ṭālibu, yā calls, ayyu is the vocative, hā alerts, and al-ṭālibu is a nominative adjective agreeing with ayyu.
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The Attention Hā in Yā Ayyuhā
Vocative Particles: Five Forms, Uses and Parsing
In yā ayyuhā al-ṭālibu, yā calls, ayyu is the vocative, hā alerts, and al-ṭālibu is a nominative adjective agreeing with ayyu.