I am trying to write a fitting routine, similar to LinearModelFit and NonlinearModelFit. I would like to generate a similar type of output, that is a FittedModel structure. It could look like this, if you apply FullForm to the result to a LinearModelFit calculation:

FittedModel[List["Linear",List[0.18644067796610153`,0.6949152542372882`],List[List[x],List[1,x]],List[0,0]],List[List[1.`,1.`,1.`,1.`]],List[List[0,1],List[1,0],List[3,2],List[5,4]],List[List[1.`,0.`],List[1.`,1.`],List[1.`,3.`],List[1.`,5.`]],Function[Null,Internal`LocalizedBlock[List[x],Slot[1]],List[HoldAll]]]

Some parts of this structure is easy to interpret, but has anyone the full interpretation?