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Check out the new article: Designing a Strategy State Machine in MQL5: Replacing Nested If-Else Logic with Formal States.
Nested if-else logic inside OnTick() creates implicit states that are hard to isolate, debug, and extend without regressions. A formal finite state machine in MQL5 uses an IState interface, a CStrategyContext mediator, and four concrete states to separate detection from behavior. A three-file include structure resolves circular dependencies and keeps declarations, definitions, and instantiation clean, making changes safer and debugging faster.
A typical Expert Advisor begins with a manageable OnTick() function. There is a condition to check whether a position is open, a condition to evaluate the entry signal, a condition to verify the spread is acceptable, and a condition to check whether the session is active. Four conditions, clearly readable, easily debugged.
After six months of feature additions, the same function grows to forty-three nested conditionals across 230 lines. To add a trailing stop, a new developer must first identify the exact condition set that indicates an open, profitable position before the first take-profit level. This requires reading the entire function, mapping the execution paths, and choosing an insertion point that does not break existing behavior. The probability of introducing a regression is not low.
The root problem is that the EA has implicit states — idle, looking for entry, in a trade, managing exit — but encodes them as scattered boolean flag combinations rather than explicit structural entities. With implicit state, state-handling code is interleaved with boundary-detection code. As a result, the function must answer two questions at each evaluation point: the current state and the action to perform. These are two distinct responsibilities, and combining them is what produces the fragility.
Author: Ushana Kevin Iorkumbul