wiki:TypeFunctionsRenaming

Version 2 (modified by chak, 8 years ago) (diff)

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Type Functions: Renaming

Phasing

GHC is organised such that class and type declarations are processed (during renaming and type checking) before any instance declarations are considered. In the presence of associated types, instance declarations may contain type definitions. In particular, the data constructors introduced by associated data declarations need to be brought into scope before we can rename any expressions.

Renaming of indexed types

Kind signatures

Kind signatures are renamed by RnSource.rnTySig, which is parametrised by a function that handles the binders (i.e., index variables) of the declaration. This is so that we can use the same code for toplevel signatures and those in classes. In the former case, the variables are in a defining position, whereas in classes they are in a usage position (as all index variables must be class parameters).

Definitions of indexed types

There is little extra that needs to be done for indexed types. The main difference between vanilla synonyms and data/newtype declarations and the indexed variants is that the tcdTyPats field is not Nothing. We simply call rnTyPats on these fields, which traverses them in the usual way.

Renaming of associated types

Associated data definitions are particularly interesting, as they not only introduces, but also value level entities, namely the data constructors. During renaming, we enter the names of all data constructors that an associated data type defines into the global RdrName environment by extending the function RnNames.getLocalDeclBinders such that it traverses instance declarations, too. We are careful not to add the data type constructor multiple times by ignoring them in instance declarations. The global RdrName environment only ever contains the type constructor introduced in the class declaration (i.e, the RdrName of an associated data type maps to the Name of the AT declaration in the class).


Remaining problem: The function getLocalDeclBinders must still supply the parent Name to the name generation for the data constructors. That parent name should be the one produced for the associated data declaration in the corresponding class declaration, which is hard to get hold of at this moment. So, we supply the Name of the data type constructor instead. That should probably be replaced by the class name in a later phase.


Otherwise, RnSource.rnSrcInstDecl invokes RnSource.rnTyClDecl on all associated types of an instance to rename them.