reorderable 0.3 → 0.3.1
raw patch · 1 files changed
+34/−34 lines, 1 files
Files
- reorderable.cabal +34/−34
reorderable.cabal view
@@ -1,5 +1,5 @@ Name: reorderable-Version: 0.3+Version: 0.3.1 Cabal-Version: >= 1.2 Build-Type: Simple License: OtherLicense@@ -12,22 +12,22 @@ Category: Type System, Data Description:- + . * Introduction.- + . The pair `(Int, Float)' is entirely distinct from the pair `(Float, Int)' and trying to use one in place of the other will give a type error. This is often, but not always, desired.- + . * Module.- + . This module provides more flexible sum and product types that do not enforce a single order on their elements. This does introduce some necessary restrictions, for example only one instance of any type can appear in any given collection of types. Additionally, all types that are to be used in one of these flexible containers must be pre-defined as `reorderable':- + . @ data MyType1 = MyType1 Int data MyType2 = MyType2 Float@@ -39,47 +39,47 @@ reorderable ''MyType3 reorderable ''MyType4 @- + . That will, using /Template Haskell/, generate all the required instances to make those types usable as reorderable types within unordered containers. Following that, all the declarations below are valid:- + . @ type Reordered1A = ReorderableEnd :*: MyType2 :*: MyType1 type Reordered1B = ReorderableEnd :*: MyType1 :*: MyType2 type Reordered2 = Reordered1A :*: MyType3 type Reordered3 = ReorderableEnd :*: MyType4 :*: Reordered1B @- + . Types `Reordered1A' and `Reordered1B' are in fact now identical. This does introduce a third limitation of the library I have been unable to lift - the use of `ReorderableEnd' as a sentinel in all reorderable containers.- + . It may be the case that `Type1' and `Type2' can be used together, as can `Type3' and `Type4', but the two sets of types can not be used in a container together. These are /groups/ of types:- + . @ reorderableGroup [''MyType1, ''MyType2] reorderableGroup [''MyType3, ''MyType4] @- + . The groups can overlap:- + . @ reorderableGroup [''MyType1, ''MyType2] reorderableGroup [''MyType1, ''MyType3, ''MyType4] @- + . But this may cause some \"leakage\" where types from two different groups (for example `MyType2' and `MyType4') end up in the same container, attached via common types.- + . * Generation.- + . For each type `X' for which `reorderable' (or equivalent) is called, the following functions are generated (where `X' is the type name):- + . @ addSumX :: (x :>: s) => s -> s :+: x setSumX :: (x :<: s) => x -> s -> s@@ -91,9 +91,9 @@ removeProductX :: (x :?: s) => s -> s :-: x @- + . ** Notes on the syntax:- + . * `:<:' Is read as \"Is member of sum type\". * `:>:' Is read as \"Is not member of sum type\". * `:+:' Is read as \"Plus\".@@ -102,49 +102,49 @@ * `:~:' Is read as \"Is not member of product type\". * `:*:' Is read as \"Product\". * `:-:' Is read as \"Remove\".- + . ** Notes on the functions:- + . * `addSumX' Adds the TYPE `x' to the given signature, and correctly re-wraps the contained data to reflect this new structure. It does not add any data in to the structure itself because only one item may exist in the structure, and that item is already there.- + . * `setSumX' Changes what data is currently stored in the sum. For a given concrete sum type `S', this can be called as: `setSumX x (undefined :: S)'. An alternative version is simply: `setSumType (undefined :: S) x', in which `X :<: S'. This is equivalent to the original `inj' function from `Data Types \'a la Carte', but has an explicit type proxy.- + . * `getSumX' Returns the data of type `Just X' IF it is the data currently being stored within the sum, otherwise it returns `Nothing'. This is equivalent to the original `prj' function from `Data Types \'a la Carte'.- + . * `addProductX' Adds data of type `X' to an existing product type that does not yet contain any data of that type.- + . * `setProductX' Sets the data of type `X' in a product type that already contains data of that type.- + . * `getProductX' Gets the data of type `X' from a product type that contains data of that type.- + . * `removeProductX' Removes data of type `X' from a product type that contains data of that type, and rewraps the resulting information to remove `X' from the product's type. There is no `removeSumX' function because the result is empty if the stored data is not of the type being removed.- + . * Generators.- + . In addition to being able to control for which types code is generated, you can control what code is generated for them through `reorderer's. Note that the default code listed above is ALWAYS generated, you can currently only ADD to the generation code. The simplest way to explain this is through an example:- + . @ class ReorderableSum a @@ -159,7 +159,7 @@ getSum??? with = getSumType with (undefined :: ???) |] @- + . The code above is exactly the code used to generate the sum type functions documented above. The internal class names are used in place of the type operator synonyms for simplicity. `???' is used as a placeholder@@ -171,7 +171,7 @@ been generated (using `reify'). The simple reason for this is that placing the same type in two `reorderableGroup's will, without that check, attempt to instantiate this code twice and thus give errors.- + . What can be done within generators is very constrained. For one thing, the parameter `a' to `ReorderableSum' currently MUST have kind `*', so any reorderable types may not have type parameters themselves (unless a new@@ -179,11 +179,11 @@ placeholder `???' in no way accounts for complex names - it is purely a text-based replacement, so trying to create a reorderable ``Maybe Int'' type will result in the illegal:- + . @ addSumMaybe Int :: ... @- + . Finally, this code is processed with \"haskell-src-meta\", and so any code must be parsable with that code. One lifting of this restriction is that reorderers may additionally contain type family declarations, which