This paper introduces Siren, a hybrid system for algorithmic composition and live-coding performances. Its hierarchical structure allows small modifications to propagate and aggregate on lower levels for dramatic changes in the musical output. It uses functional programming language TidalCycles as the core pattern creation environment due to its inherent ability to create complex pattern relations with minimal syntax. Borrowing the best from TidalCycles, Siren augments the pattern creation process by introducing various interface level features: a multi-channel sequencer, local and global parameters, mathematical expressions, and pattern history. It presents new opportunities for recording, refining, and reusing the playback information with the pattern roll component. Subsequently, the paper concludes with a preliminary evaluation of Siren in the context of user interface design principles, which originates from the cognitive dimensions framework for musical notation design.