README (#3)

README.md

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+# Railway Result
+
+Implementation of a **Result Pattern** for C# with functional extension methods leveraging **Railway Oriented Programming**. Learn more about [Railway Oriented Programming](https://fsharpforfunandprofit.com/rop/).
+
+Extension Methods:
+
+- `Ensure` and `EnsureNotNull` - tests for condition resulting in succedded/failed result object (railway).
+- `Then` - maps current value of result object to a new value (on succeeded railway).
+- `Tap` - invokes funcion on value of result object (on succeeded railway).
+- `And` - encapsulates additional value into result object (on succeeded railway).
+
+### Example:
+```csharp
+public Result SetMemberRole(Guid initiatorId, Guid memberId, TeamRole newRole)
+{
+   return newRole
+      .Ensure(role => !role.IsOwner(), Errors.CannotHaveMultipleTeamOwners)
+      .Then(_ => GetTeamMemberByUserId(initiatorId))
+      .Ensure(member => member.Role.CanUpdateTeamRoles(), Errors.ForbiddenToUpdateTeamRoles)
+      .Then(_ => GetTeamMember(memberId))
+      .Ensure(member => !member.Role.IsOwner(), Errors.ForbiddenToChangeTeamOwnersRole)
+      .Tap(member => member.UpdateRole(newRole))
+      .ToResult();
+}
+
+public Result<TeamMember> GetTeamMemberByUserId(Guid userId)
+{
+   var member = _members.Find(member => member.UserId == userId);
+   return member.EnsureNotNull(Errors.NotMemberOfTeam);
+}
+
+public Result<TeamMember> GetTeamMember(Guid memberId)
+{
+   var member = _members.Find(member => member.Id == memberId);
+   return member.EnsureNotNull(Errors.MemberNotFound);
+}
+```
+
+### Result Pattern without using railway-oriented programing:
+```csharp
+public Result SetMemberRole(Guid initiatorId, Guid memberId, TeamRole newRole)
+{
+   if (newRole.IsOwner())
+      return Errors.CannotHaveMultipleTeamOwners;
+
+   var initiator = GetTeamMemberByUserId(initiatorId);
+   if (initiator is null)
+      return Errors.NotMemberOfTeam;
+
+   if (!initiator.Role.CanUpdateTeamRoles())
+      return Errors.ForbiddenToUpdateTeamRoles;
+
+   var member = GetTeamMember(memberId);
+   if (membe is null)
+      return Errors.MemberNotFound;
+
+   if (member.Role.IsOwner())
+      return Errors.ForbiddenToChangeTeamOwnersRole;
+
+   member.UpdateRole(newRole);
+   return Result.Success;
+}
+
+public TeamMember? GetTeamMemberByUserId(Guid userId)
+{
+   return _members.Find(member => member.UserId == userId);
+}
+
+public TeamMember? GetTeamMember(Guid memberId)
+{
+   return _members.Find(member => member.Id == memberId);
+}
+```
+
+*Note: using railway-oriented programming results in cleaner and shorter code, however, it also brings additional overhead and inability to return early from function, thus making it less performant.*
+
+*Learn more about [when to not use railway-oriented programming](https://fsharpforfunandprofit.com/posts/against-railway-oriented-programming/).*