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The express problem is solved, and some of the solutions are given in the article. Tags: Mobile edit Mobile web edit |
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However, if we wish to add a new function over the type (a new method in C# terminology), for example to pretty print an expression, we have to change the {{code|IEvalExp}} interface and then modify all the classes that implement the interface. Another possibility is to create a new interface that extends the {{code|IEvalExp}} interface and then create sub-types for {{code|Lit}}, {{code|Add}} and {{code|Mult}} classes, but the expression returned in {{code|ExampleOne.AddOneAndTwo()}} has already been compiled so we will not be able to use the new function over the old type. The problem is reversed in functional programming languages like [[F Sharp (programming language)|F#]] where it is easy to add a function over a given type, but extending or adding types is difficult.
=== Problem solution using object algebra ===
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