| 84 | | == Kinds == |
| 85 | | |
| 86 | | Kinds are represented as types: |
| 87 | | {{{ |
| 88 | | type Kind = Type |
| 89 | | }}} |
| 90 | | Basic kinds are now |
| 91 | | represented using type constructors, e.g. the kind `*` is represented as |
| 92 | | {{{ |
| 93 | | liftedTypeKind :: Kind |
| 94 | | liftedTypeKind = TyConApp liftedTypeKindTyCon [] |
| 95 | | }}} |
| 96 | | where `liftedTypeKindTyCon` is a built-in `PrimTyCon`. The arrow type |
| 97 | | constructor is used as the arrow kind constructor, e.g. the kind `* -> *` |
| 98 | | is represented internally as |
| 99 | | {{{ |
| 100 | | FunTy liftedTypeKind liftedTypeKind |
| 101 | | }}} |
| 102 | | It's easy to extract the kind of a type, or the sort of a kind: |
| 103 | | {{{ |
| 104 | | typeKind :: Type -> Kind |
| 105 | | }}} |
| 106 | | The "sort" of a kind is always one of the |
| 107 | | sorts: `TY` (for kinds that classify normal types) or `CO` (for kinds that |
| 108 | | classify coercion evidence). The coercion kind, `T1 :=: T2`, is |
| 109 | | represented by `PredTy (EqPred T1 T2)`. |
| 110 | | |
| 111 | | === Kind subtyping === |
| 112 | | |
| 113 | | [[Image(https://docs.google.com/drawings/pub?id=1M5yBP8iAWTgqdI3oG1UNnYihVlipnvvk2vLInAFxtNM&w=359&h=229)]] |
| 114 | | |
| 115 | | (You can edit this picture [https://docs.google.com/drawings/d/1M5yBP8iAWTgqdI3oG1UNnYihVlipnvvk2vLInAFxtNM/edit?hl=en_GB here].) |
| 116 | | |
| 117 | | `*` is the kind of boxed values. Things like `Int` and `Maybe Float` have kind `*`. |
| 118 | | |
| 119 | | `#` is the kind of unboxed values. Things like `Int#` have kind `#`. |
| 120 | | |
| 121 | | `(#)` is the kind of unboxed tuples. Things like `(# Int, Int #)` have kind `(#)`. |
| 122 | | |
| 123 | | `ArgKind` is the kind of things that can appear as arguments to functions. |
| 124 | | |
| 125 | | `OpenKind` is the kind of things that can appear as results of functions. |
| 126 | | |
