This paper explores the concept of end-user programming languages in music composition, and introduces the Manhattan system, which integrates formulas with a grid-based style of music sequencer. Following the paradigm of spreadsheets, an established model of end-user programming, Manhattan is designed to bridge the gap between traditional music editing methods (such as MIDI sequencing and typesetting) and generative and algorithmic music -seeking both to reduce the learning threshold of programming and support flexible integration of static and dynamic musical elements in a single work. Interaction draws on rudimentary knowledge of mathematics and spreadsheets to augment the sequencer notation with programming concepts such as expressions, built-in functions, variables, pointers and arrays, iteration (for loops), branching (goto), and conditional statements (if-then-else). In contrast to other programming tools, formulas emphasise the visibility of musical data (e.g. notes), rather than code, but also allow composers to interact with notated music from a more abstract perspective of musical processes. To illustrate the function and use cases of the system, several examples of traditional and generative music are provided, the latter drawing on minimalism (process-based music) as an accessible introduction to algorithmic composition. Throughout, the system and approach are evaluated using the cognitive dimensions of notations framework, together with early feedback for use by artists.