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| − | [[Category: ECMA]] | + | Replaced by [[restrict]] |
| − | [[Category:Catcall]]
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| − | {{Research}}
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| − | {{UnderConstruction}}
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| − | | + | |
| − | ==Introduction==
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| − | By allowing covariant feature redefinition and hiding of features, the Eiffel language introduces the problem of cat-calls (Changed Availability or Type). The forget / keep mechanism prevents this by introducing new derived types whenever a feature is covariantly redefined or the export status is restricted. These derived types are then used to prevent cat-calls through the conformance rules to the original types.
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| − | This mechanism is a variant on forget/keep as originally envisaged by Mark Howard. This variant was worked out by us when he explained to me his ideas about forget/keep.
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| − | ===Example classes===
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| − | The following classes will be used for illustration:
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| − | {| class="top-aligned"
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| − | |
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| − | <e>
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| − | class ANIMAL
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| − | | + | |
| − | feature
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| − | | + | |
| − | eat (f: FOOD)
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| − | | + | |
| − | sleep
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| − | | + | |
| − | end
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| − | </e>
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| − | |
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| − | <e>
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| − | class CAT
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| − | | + | |
| − | inherit
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| − | | + | |
| − | ANIMAL
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| − | redefine
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| − | eat
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| − | export
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| − | {NONE} sleep
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| − | end
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| − | | + | |
| − | feature
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| − | | + | |
| − | eat (f: CAT_FOOD)
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| − | | + | |
| − | end
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| − | </e>
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| − | |
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| − | <e>
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| − | class FOOD
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| − | | + | |
| − | end
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| − | | + | |
| − | | + | |
| − | class CAT_FOOD
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| − | | + | |
| − | inherit
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| − | | + | |
| − | FOOD
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| − | | + | |
| − | end
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| − | </e>
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| − | |
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| − | <e>
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| − | class LIST [G]
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| − | | + | |
| − | feature
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| − | | + | |
| − | has (g: G): BOOLEAN
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| − | | + | |
| − | put (g: G)
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| − | | + | |
| − | item: G
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| − | | + | |
| − | end
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| − | </e>
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| − | |}
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| − | | + | |
| − | ===The problem===
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| − | The problem of cat-calls has two variants, one because a feature is covariantly redefined:
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| − | | + | |
| − | <e>
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| − | local
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| − | a: ANIMAL
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| − | c: CAT
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| − | food: FOOD
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| − | do
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| − | | + | |
| − | a := c
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| − | -- illegal since `eat' for type CAT takes arguments only of type CAT_FOOD
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| − | a.eat (food)
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| − | | + | |
| − | end
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| − | </e>
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| − | | + | |
| − | And one because the visibility of features change:
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| − | <e>
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| − | local
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| − | a: ANIMAL
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| − | c: CAT
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| − | do
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| − | | + | |
| − | a := c
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| − | -- `sleep' is hidden for type CAT and should not be called
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| − | a.sleep
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| − | | + | |
| − | end
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| − | </e>
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| − | | + | |
| − | ===The idea===
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| − | To solve the problem of cat-calls, the idea is that whenever you covariantly redefine a feature of a parent type, you don't actually inherit this feature and redefine it, but you inherit from a (virtual) parent type which does not have this feature at all. The same goes for changing the export status. When you restrict the export status of a feature, you don't inherit this feature from the parent but introduce it in the current type and inherit from a parent which does not know this feature at all.
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| − | As a consequence of this, the types as we know them don't necessarily conform to each other. If the type <e>CAT</e> inherits from the type <e>ANIMAL</e> but covariantly redefines the feature <e>eat</e> than the type <e>CAT</e> does not conform to <e>ANIMAL</e> anymore, but only to a type <e>ANIMAL</e> without the feature <e>eat</e>.
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| − | | + | |
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| − | ==Without mechanism==
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| − | The without mechanism introduces a new keyword <e>without</e> which can be used to remove covariantly redefined features and features which change the export status from a base type. By removing features, a new type is introduced. The new type has all the features that the original type had, except those listed in the without clause. If <e>without all</e> is used, all features in the original will be removed from the new type. This produces a type with no features at all, which isn't very useful on its own. But it is useful if you want to lose all except a few of the features. We intoduce another new keyword <e>except</e> which can only follow <e>without all</e> (therefore it does not need to be a compeletly reserved word).
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| − | ===Default behavior===
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| − | In the without mechanism the normal declaration of a type will have all the features of a type from which it inherits. This implies that all subtypes which change an export status or covariantly redefine a feature don't conform to this type anymore.
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| − | <e>
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| − | local
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| − | -- normal declaration means we have all the features
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| − | animal: ANIMAL
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| − | cat: CAT
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| − | do
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| − | -- legal since the feature `eat' is in the normal type
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| − | animal.eat (food)
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| − |
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| − | -- legal since the feature `sleep' is in the normal type
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| − | animal.sleep
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| − | | + | |
| − | -- illegal since CAT does not conform to ANIMAL anymore.
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| − | -- N.B. The declaration of CAT given above is illegal under this proposal.
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| − | -- Adding the without keyword will make it legal again, and emphasize the lack of conformance.
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| − | animal := cat
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| − | end
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| − | </e>
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| − | | + | |
| − | ===Syntax===
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| − | <e>
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| − | -- normal declaration is an object with all features and no
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| − | -- subtype which has a covariantly redefined feature conforms to this type
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| − | a1: ANIMAL
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| − | -- a type which lacks all features, thus all
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| − | -- subtypes conform to this type. It is not a useful type.
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| − | a2: ANIMAL without all end
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| − | -- a type which only lacks the features `eat' and `sleep'. All subtypes which only redefine or change export status
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| − | -- of `eat' or `sleep' will conform to this type
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| − | a3: ANIMAL without eat, sleep end
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| − | -- a type which lacks all features other than eat.
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| − | a4: ANIMAL without all except eat end
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| − | </e>
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| − | ==Covariant feature redefinition==
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| − | ===Cat-call problem===
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| − | With covariant feature redefinition you run into cat-call problems as this example shows:
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| − | <e>
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| − | local
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| − | a: ANIMAL
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| − | c: CAT
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| − | food: FOOD
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| − | do
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| − | a := c
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| − | -- eat for type CAT takes arguments only of type CAT_FOOD
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| − | a.eat (food)
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| − | end
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| − | </e>
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| − | | + | |
| − | ===Without mechanism===
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| − | Using the without mechanism the default behavior for a type is to keep all features. Types which want to have covariant redefined features will not conform to the original type. In fact, under this scheme, you do not "redefine" features covariantly at all. As you are not inheriting them from the original type (specified by the without keyword), you simply introduce them in the class text. You do not mention them in a redefine clause.
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| − | {| class="top-aligned"
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| − | <e>
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| − | class CAT
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| − | | + | |
| − | inherit
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| − | | + | |
| − | ANIMAL
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| − | without eat end
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| − |
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| − | | + | |
| − | feature
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| − | | + | |
| − | eat (f: CAT_FOOD)
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| − | | + | |
| − | end
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| − | </e>
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| − | |}
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| − | | + | |
| − | Now the cat-call example with the new without types:
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| − | | + | |
| − | <e>
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| − | local
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| − | a: ANIMAL
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| − | c: CAT
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| − | do
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| − | -- illegal assignment, ANIMAL and CAT don't conform
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| − | -- since CAT inherits from ANIMAL without eat
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| − | a := c
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| − | a.eat (food)
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| − | | + | |
| − | end
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| − | </e>
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| − | | + | |
| − | ==Feature hiding==
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| − | | + | |
| − | ===Cat-call problem===
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| − | By restricting the export status of features, a cat-call problem is introduced:
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| − | <e>
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| − | local
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| − | a: ANIMAL
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| − | c: CAT
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| − | do
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| − | | + | |
| − | a := c
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| − | -- sleep is hidden for type CAT and should not be called
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| − | a.sleep
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| − | end
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| − | </e>
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| − | | + | |
| − | ===Using without mechanism===
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| − | Using the without mechanism the default behavior for a type is to keep all features. Types which change the export status of features will not conform to the original type. This is not allowed. Instead you use the without mechanism to remove these features from the type you are creating, then introduce them afresh in an export {NONE} feature clause, like any other new feature.
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| − | | + | |
| − | {| class="top-aligned"
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| − | |-
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| − | ! the old way
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| − | ! the new way
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| − | |-
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| − | <e>
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| − | class CAT
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| − | | + | |
| − | inherit
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| − | | + | |
| − | ANIMAL
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| − | export
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| − | {NONE} sleep
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| − | end
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| − | end
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| − | </e>
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| − | |
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| − | <e>
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| − | class CAT
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| − | inherit
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| − | | + | |
| − | ANIMAL
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| − | without sleep end
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| − | feature {NONE} -- Implementation
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| − |
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| − | sleep is
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| − | -- ...
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| − | do
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| − | ...
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| − | end
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| − | | + | |
| − | end
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| − | </e>
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| − | |}
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| − | Now the cat-call example:
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| − | | + | |
| − | <e>
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| − | local
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| − | a: ANIMAL
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| − | c: CAT
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| − | do
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| − | -- illegal assignment, ANIMAL and CAT don't conform
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| − | a := c
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| − | a.sleep
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| − | end
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| − | </e>
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| − | <e>
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| − | local
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| − | a: ANIMAL without all except eat end
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| − | c: CAT
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| − | do
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| − | -- legal, CAT conforms to ANIMAL without all except eat
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| − | a := c
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| − | -- illegal, ANIMAL without all except eat doesn't have a feature sleep
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| − | a.sleep
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| − | end
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| − | </e>
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| − | ==Generics==
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| − | Conformance of generic classes (two generic classes conform if their base classes conform and their generic parameters conform) introduces another kind of covariantly redefined features. Every feature which has a generic argument, e.g. <e>put (g: G)</e> in class <e>LIST [G]</e> can be regarded as covariantly redefined since <e>put</e> in <e>LIST [ANY]</e> takes an argument of type <e>ANY</e> and <e>put</e> in <e>LIST [ANIMAL]</e> takes an argument of type <e>ANIMAL</e>. But <e>LIST [ANIMAL]</e> conforms to <e>LIST [ANY]</e>, thus the feature <e>put</e> is actually covariantly redefined in <e>LIST [ANIMAL]</e>.
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| − | ===Cat-call problem===
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| − | The conformance rules of generics also introduce cat-call problems:
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| − | <e>
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| − | local
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| − | -- feature `put' has argument type ANY
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| − | any_list: LIST [ANY]
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| − | -- feature `put' has argument type ANIMAL
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| − | animal_list: LIST [ANIMAL]
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| − | do
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| − | -- animal_list conforms to any_list, but arguments of the feature
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| − | -- `put' are different
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| − | any_list := animal_list
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| − | | + | |
| − | -- since any type can be put into any_list this will add
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| − | -- an integer to the animal_list
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| − | any_list.put (5)
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| − | end
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| − | </e>
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| − | | + | |
| − | ===Conformance===
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| − | A generic class X [G] conforms to another generic class X [H] if G and H are the same class, or if G conforms to H and H lacks any features taking generic arguments.
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| − |
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| − | ===Using without mechanism===
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| − | <e>
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| − | local
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| − | any_list: LIST [ANY]
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| − | animal_list: LIST [ANIMAL]
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| − | do
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| − | -- illegal since animal_list does not conform to any_list anymore
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| − | any_list := animal_list
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| − | | + | |
| − | any_list.put (5)
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| − | | + | |
| − | end
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| − | </e>
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| − | | + | |
| − | <e>
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| − | | + | |
| − | local
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| − | any_list: like usable_list
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| − | animal_list: LIST [ANIMAL]
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| − | animal: ANIMAL
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| − | do
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| − | -- legal since LIST [ANIMAL] has `put'
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| − | animal_list.put (animal)
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| − | | + | |
| − | -- legal since animal_list conforms to `usable_list'
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| − | any_list := animal_list
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| − | | + | |
| − | -- illegal since `usable_list' does not have `put'
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| − | any_list.put (animal)
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| − | -- illegal, since `usable_list' does not have `has'
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| − | any_list.has (animal)
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| − | | + | |
| − | end
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| − | | + | |
| − | feature {NONE} -- Anchors
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| − | usable_list: LIST [ANY] without put, has, append, etc. etc. end
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| − | -- Anchor - in practice this anchor is defined in a class which is imported using non-conforming inheritance
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| − | </e>
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| − | | + | |
| − | --Colin Adams and Mark Howard
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