Type-Safe Limited Collections




Dustin Voss




Standards Track








19-Dec-2013, 28-Jul-2013




To improve type safety of collections and particularly limited collections, this DEP adds a default-fill: keyword argument to limited collection classes and an element-type-fill and element-type getter to all collections.


The collection classes that allow a fill: keyword argument are:

  • <array>

  • <vector>

  • <simple-vector>

  • <simple-object-vector>

  • <stretchy-vector>

  • <deque>

  • <list>

  • <string>

  • <byte-string>

  • <unicode-string>

This DEP refers to these classes as fillable collection classes. Fillable collection classes are comprised of all the direct and indirect subclasses of <mutable-sequence> excepting <mutable-sequence> itself, <pair>, and <empty-list>.


The limited methods on fillable collection classes will each add the following additional parameter:




#key default-fill :: <character> = ' '


#key default-fill :: K1 = C1


#key default-fill :: K2 = C2

All others

#key default-fill :: <object> = #f

K1 and K2 are unspecified subclasses of <character>. C1 and C2 are unspecified values equivalent to a space character.

The limited methods on fillable collection classes each return a limited type designated L. The fill: init-keyword of all L will have a default value identical to the default value listed above instead of #f.

The type-for-copy method on instances of L will return a limited type L2 as described in the [DRM], but with the additional requirement that L2 will have the same default fill value as L (i.e., the same default-fill: argument).

The defaulted or supplied default-fill: argument should be an instance of the element type of the collection. However, it is not an error to supply a default fill value that the collection cannot accommodate, since a call to make may specify a fill value of the correct type.


A new open generic function will be added to the Dylan library:

element-type-fill (coll :: <collection>) => (fill-value :: <object>)

For instances of a limited fillable collection class type, this function will return the (possibly defaulted) default-fill: argument to limited. For instances of all other fillable collection classes, this function will return the following:


Return Value


' '





All others


C1 and C2 are unspecified values equivalent to a space character.

It is an error to call element-type-fill on an instance of a collection class that does not support the fill: init-keyword. However, implementations may find this difficult to check (because users can introduce that keyword in any subclass) and may return #f for such collections instead. Portable programs should not rely on this behavior.


A new open generic function will be added to the Dylan library:

element-type (coll :: <collection>) => (element-type :: <type>)

For instances of collection classes with a limited element type, this function will return that element type. For instances of other collection classes, this function will return the element type described in Element Types of the [DRM].


The second paragraph of the Collection Operations section of the [DRM] states the following:

Note to implementors: Functions such as map, map-as that return a new collection cannot rely on the type they instantiate having a valid default for fill:. Therefore, when the size of the result is nonzero, these functions should compute the first element of the result before making the collection and specify that element as the fill: value. Otherwise a spurious type error could occur when making the collection.

However, there is a problem with the size-setter method that is not addressed by the above note. That method may be called on an empty collection to grow it. The DRM states:

The value of each new element is the same as would have been used if the stretchy sequence had been created with make, specifying size: new-size but not fill:.

That is, new elements are the default fill: value for the collection. This will be #f, 0, or ' ' depending on the type of limited collection. But in a user-defined limited collection, such as limited(<vector>, of: <shape>), the default causes a spurious type error. And if the collection is empty, the workaround described in the DRM of using the first element of the collection cannot be used.

This DEP solves that problem by describing a way for size-setter to populate a collection with valid values. This DEP also improves the interface/implementation separation of limited collections by allowing a library author to specify a valid default for fill: in a type-defining limited call rather than requiring the client to know and use a valid fill: value in every call to make.

Additionally, this DEP adds the element-type method. This method is useful for code that transforms or manipulates one collection into a different form. The example of the <stream> classes comes to mind. If you write code that maps a stream to or from a user-supplied collection, that code cannot verify compatibility between the stream’s stream-element-type and the collection’s element type. Adding the element-type method solves that problem.


I named element-type-fill as such rather than default-fill because the latter name is a little more misleading. A user can define a subclass of a collection and provide a new default value for the fill: init-keyword without needing to define a new element-type-fill method; they only need to do that when restricting the element type of a collection.

The element-type-fill and element-type methods take an instance of a collection class as an argument rather than the type of the collection. This is necessary because the [DRM] allows the limited function on C to return C itself as a type, implying that the default fill and element type information associated with the limited collection has to be available on a per-instance basis. Plus, creating getters on types is not idiomatic to Dylan.

A previous draft had changed the behavior of map, etc., so that they would instantiate their resulting collection with the collection’s default fill value. This turned out to be problematic. Most limited collections used in the Open Dylan source code do not have a false-or element type, and the element type they do have lacks a sensible “blank” value such as #f, 0, or "" to use as a default fill. Strictly speaking, such a collection is poorly formed because it can never be directly instantiated with a specific size; however, that situation never occurs in the Open Dylan source code. Instead, the source code instantiates such a collection indirectly via map or as. Those functions fill the collection with a value derived from the first element of the argument(s), as described by the DRM’s note to implementors quoted above. The value is not the “correct” value for any element but the first, but of course each other element is given its own “correct” value immediately thereafter, with the fill value merely acting as a placeholder. Under the previous draft this fill behavior would have been removed and all those limited collections would have needed to be changed; in order to tolerate being instantiated “empty”, they would have needed a false-or element type. That would have been a significant lapse in backwards compatibility with the Open Dylan source code and presumably with other source code as well. Therefore, I revised this DEP to leave map, etc., functioning as they always have. Those “poorly-formed” limited collections are still poorly formed, but…they work in their environment.

I had originally considered a more extensive change where each instance of a fillable collection class would not only track its default fill value, but also track the specific fill: value that it was created with. But in thinking about it, I feel the designers made the right call in leaving that information out of each instance. In particular, the implementation of <list> would be difficult if each instance tracked its fill: value.

Implementation Notes


The Open Dylan implementation already defines this internally. The name just needs to be exported.

Backwards Compatibility

This DEP does not change the limited collection type relationships described in the Limited Collection Types section of the [DRM].

Before this DEP, the Open Dylan implementation of limited collections effectively specified a default-fill: argument for certain combinations of collection and element type, as follows:


Element Types


<byte>, <double-byte>, <machine-word>, <integer>, <single-float>, <double-float>


<byte>, <double-byte>, <machine-word>, <integer>, <single-float>, <double-float>


<byte>, <byte-character>

Programs that relied on this behavior should instead specify either the default-fill: argument to limited or the fill: init-keyword to make.

An additional side-effect of this relates to the subclass function in type specializers. The subclass function does not work with every limited collection type, because limited collection types are not classes. That is, limited(<vector>, of: <color>) is not congruent to subclass(<vector>), and never has been. However, subclass will work with the above combinations of collection and element type, assuming the correct default-fill: is provided. That is, limited(<vector>, of: <integer>, default-fill: 0) is congruent to subclass(<vector>).

Existing subclasses of <collection> that define their own fill: init-keyword will still work, assuming they also specify a default value for that keyword that is of the element type of the subclass.

New code may use element-type or element-type-fill in conjunction with an existing subclass of <collection> that does not define those methods but nonetheless has restricted element types. element-type and element-type-fill will then return <object> and #f, which may not be correct for that collection’s allowed element types.

The only other backwards compatibility issue is a namespace collision if the user defines their own unrelated “element-type” or “element-type-fill” bindings.

Reference Implementation

The reference implementation includes the following implementation-specified behavior:

  • limited(<string>, …) returns a limited(<byte-string>, …).

  • element-type-fill on an instance of a non-fillable collection class returns #f (as opposed to signaling an error).

The reference implementation currently includes an element-type-fill slot in every instance of a <simple-T-X> class (e.g. <simple-byte-vector>). Ideally, the <simple-T-X> class would hard code a common element-type-fill value like 0 or ' ', and an additional subclass <simple-T-X-with-fill> would include the element-type-fill slot if the user wants a more unusual fill value. Unfortunately, it appears that the implementation of repeated slots does not allow for subclasses of a class with repeated slots.

A possible remedy is to implement two subclasses of a <simple-T-X> class: <simple-T-X-common-fill> and <simple-T-X-custom-fill>. <simple-T-X> would become abstract and the two concrete subclasses would define the repeated slot. The code specifically allows for this in the dfmc-modeling module’s limited-element-type-mappings-definer macro; it can return a different concrete class depending on the default-fill: argument to limited.

[DRM] (1,2,3,4,5)

Dylan Reference Manual