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Read type class ported to C#
// Read type class ported to C#
// Only tested with: Mono C# compiler version 2.4.2.3
//
// Because there are no static interface members, new instances of the
// Dictionary classes have to be created (although they contain no state).
//
// The functions using the read method (like mulStrings) explicitly mention
// `int` in IReadDictionary<int>, but that's just like `x = read "4" :: Int` in
// Haskell.
//
// Also, since .NET doesn't allow for existing (sealed) classes to be extended,
// the Dictionary classes are separate from the types they work on (can't make
// `class System.Int32 : IReadDictionary<Int32>`).
using System;
// class Read a where
// read :: String -> a
interface IReadDictionary<A>
{
A read(string s);
}
class ReadTest
{
public static void Main()
{
// Explictly pass int dictionary
int x = mulStrings(new IntDictionary());
Console.WriteLine(x);
// Explictly pass bool dictionary
bool b = andStrings(new BoolDictionary());
Console.WriteLine(b);
// Polymorphic usage
int y = readSomething(new IntDictionary());
Console.WriteLine(y);
}
static int mulStrings(IReadDictionary<int> dict)
{
int x = dict.read("4");
int y = dict.read("16");
return x * y;
}
static bool andStrings(IReadDictionary<bool> dict)
{
bool b1 = dict.read("false");
bool b2 = dict.read("true");
return b1 && b2;
}
// This function is polymorphic in A
// Here the IReadDictionary<A> interface shows its benefit over manually
// calling Int32.Parse or Boolean.Parse.
static A readSomething<A>(IReadDictionary<A> dict)
{
return dict.read("815");
}
}
class IntDictionary : IReadDictionary<int>
{
public int read(string s)
{
return Int32.Parse(s);
}
}
class BoolDictionary : IReadDictionary<bool>
{
public bool read(string s)
{
return Boolean.Parse(s);
}
}
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