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In response to a comment from @UriAgassi on my previous answer, I've devised this variant that distinguishes between retrieving constraints and solving for unknowns.

The naming convention is:

• a= to set a constraint
• a to retrieve a constraint
• a? to solve for an unknown

It violates the naming convention that ? marks a predicate, but works well for this domain-specific language.

require 'cmath'
class QuadraticEquation
 attr_accessor :a, :b, :c, :x
 def x?
 positive = (-b + CMath.sqrt(b ** 2 - 4 * a * c)) / (2 * a)
 negative = (-b - CMath.sqrt(b ** 2 - 4 * a * c)) / (2 * a)
 [positive, negative]
 end
 def a?
 (-b * x - c) / (x ** 2)
 end
 def b?
 (-a * x ** 2 - c) / x
 end
 def c?
 -a * x ** 2 - b * x
 end
end
class RightTriangle
 attr_accessor :a, :b, :c
 def a?
 Math.sqrt(c ** 2 - b ** 2)
 end
 def b?
 Math.sqrt(c ** 2 - a ** 2)
 end
 def c?
 Math.sqrt(a ** 2 + b ** 2)
 end
 alias :hypotenuse :c
 alias :hypotenuse= :c=
 alias :hypotenuse? :c?
end

One improvement is that the code above isn't littered with @ sigils. However, the main benefit of disambiguating between retrieving constraints and solving for unknowns can be observed in an over-constrained situation:

irb(main):001:0> require 'formula'
=> true
irb(main):002:0> triangle = RightTriangle.new
=> #<RightTriangle:0x007f90b401f800>
irb(main):003:0> triangle.a, triangle.b, triangle.c = 5, 12, 37
=> [5, 12, 37]
irb(main):004:0> triangle.a?
=> 35.0
irb(main):005:0> triangle.c?
=> 13.0

In response to a comment from @UriAgassi on my previous answer, I've devised this variant that distinguishes between retrieving constraints and solving for unknowns.

The naming convention is:

• a= to set a constraint
• a to retrieve a constraint
• a? to solve for an unknown

It violates the naming convention that ? marks a predicate, but works well for this domain-specific language.

require 'cmath'
class QuadraticEquation
 attr_accessor :a, :b, :c, :x
 def x?
 positive = (-b + CMath.sqrt(b ** 2 - 4 * a * c)) / (2 * a)
 negative = (-b - CMath.sqrt(b ** 2 - 4 * a * c)) / (2 * a)
 [positive, negative]
 end
 def a?
 (-b * x - c) / (x ** 2)
 end
 def b?
 (-a * x ** 2 - c) / x
 end
 def c?
 -a * x ** 2 - b * x
 end
end
class RightTriangle
 attr_accessor :a, :b, :c
 def a?
 Math.sqrt(c ** 2 - b ** 2)
 end
 def b?
 Math.sqrt(c ** 2 - a ** 2)
 end
 def c?
 Math.sqrt(a ** 2 + b ** 2)
 end
 alias :hypotenuse :c
 alias :hypotenuse= :c=
 alias :hypotenuse? :c?
end

One improvement is that the code above isn't littered with @ sigils. However, the main benefit of disambiguating between retrieving constraints and solving for unknowns can be observed in an over-constrained situation:

irb(main):001:0> require 'formula'
=> true
irb(main):002:0> triangle = RightTriangle.new
=> #<RightTriangle:0x007f90b401f800>
irb(main):003:0> triangle.a, triangle.b, triangle.c = 5, 12, 37
=> [5, 12, 37]
irb(main):004:0> triangle.a?
=> 35.0
irb(main):005:0> triangle.c?
=> 13.0