Category Theory: Function Equality by Composition Clarification

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I am new to category theory, so apologies if this is obvious.

I recently learned about function equality via composition:

∀c∈C·∀g,h : c→a If f∘g = f∘h ⇒ g=h Where C →g→ A →f→ B & C →h→ A →f→ B

This kind of makes sense, but only for the specific function f. From what I can tell, functions g & h could be considered identical in certain situations, for instance:

  • g = sin(x)
  • h = i*sin(x)
  • f = abs(x)

Clearly other functions of f could distinguish between the two functions. I'm sure I've misunderstood something but am unsure as to where.

Thanks in advance.