Changes between Version 25 and Version 26 of TypeFunctionsTypeChecking
- Aug 22, 2006 7:30:10 PM (10 years ago)
v25 v26 58 58 data AlgTyConParent = NoParentTyCon 59 59 | ClassTyCon Class 60 | FamilyTyCon TyCon 60 | FamilyTyCon TyCon -- family tycon 61 TyCon -- representation coercion 61 62 }}} 62 which is a generalisation of the old field `algTcClass` of the internal representation for datatypes, `TyCon.AlgTyCon`. In contrast to the old `algTcClass` field, the new field also appears in `IfaceSyn.IfaceDecl`. However, it does so as `Maybe IfaceTyCon` as we still do not want to represent class parent information in interfaces. 63 which is a generalisation of the old field `algTcClass` of the internal representation for datatypes, `TyCon.AlgTyCon`. In contrast to the old `algTcClass` field, the new field also appears in `IfaceSyn.IfaceDecl`. However, it does so as `Maybe IfaceTyCon` as we still do not want to represent class parent information in interfaces and we only record the family tycon in interfaces, not the coercion. (The latter is implicitly reconstructed upon loading an interface.) 64 65 NB: The type argument variables of the representation tycon are the free variables of the instance types; i.e., the representation data type is an ordinary data type, it is neither indexed nor open. The only give away of its special purpose is the value in `algTcParent`. 63 66 64 67 ==== The `DataCon`s of the variants of an instance ==== … … 70 73 the instance types are `[(a, b), v]`. Whenever the field `dcInstTys` of a `DataCon` is not `Nothing`, the field `algTcParent` of its `TyCon` must be of the form `FamilyTyCon famTyCon`, where `famTyCon` is the `TyCon` of the data family to which the instance belongs. The exact same information goes into the interface representation `IfaceSyn.IfaceConDecl` with the new field `ifConInstTys`. 71 74 72 ==== The equality axiomidentifying family instance and representation type ==== 75 ==== The identifying family instance and representation type ==== 73 76 74 === Representation of type equation axioms === 77 As each `data instance` is ''represented'' by its own `TyCon`, we need to be able to move between the type of the family instance and that of the representation. We do so by an adaptation of the same method used to implement newtypes with coercions (c.f., IntermediateTypes). Newtypes store the coercion moving between representation and abstract type in the field `nt_co` of the `NewTyCon` variant of `TyCon.AlgTyConRhs`, whereas representation types for indexed data types use `algTcParent` (see above). Newtype coercions are constructed by `Coercion.mkNewTypeCoercion`, whereas representation types for indexed data types use a similar function `Coercion.mkDataInstCoercion`, which is invoked by `BuildTyCl.buildAlgTyCon` iff it is passed family and type instance information. 78 79 === Representation of equality axioms === 75 80 76 81 Type functions have a number of properties in common with class instances; in particular, they require a machinery for matching type patterns against types, as instance heads do during context simplification. Hence, we probably want some structure similar to `InstEnv.Instance` for type functions - for instances this is maintained in the field `iSpec` of `TcEnv.InstInfo` (for type functions we don't need anything like `iBinds` as they are pure type-level entities). If possible, it would be ideal if we can reuse (or generalise) some of the matching machinery for instance heads.