List Constructor

In the ATerm representation lists are represented as terms of the form [t1,...,tn]. This is also the syntax for lists in Stratego; extended with the notation [t1,...,tn|tl] for indicating a tail of the list. This extension is necessary to take construct and deconstruct lists of arbitrary length.

In the current implementation (upto StrategoRelease064) lists are represented internally by means of the contructors Cons and Nil. Thus [t1,...,tn] is represented internally as Cons(t1,...,Cons(tn,Nil)). As a consequence ATerms are translated at every interface between internal and external representation (reading and writing).

In the new ImplementationScheme introduced in StrategoRelease06 it has become feasible to directly use the ATerm representation for lists. This should be beneficial for performance.

This requires that lists are only constructed using the ATerm Cons and Nil operations. In the old representation it is possible to use other constructors for constructing lists, such as Conc, which can be useful.

-- EelcoVisser - 04 Jan 2002

Starting with StrategoRelease07 lists will be constructed according to the scheme described above. An attempt to construct a term of the form [x | xs] where xs is not a proper list results in failures.

The special status of lists affects all generic term manipulation operations such as generic traversal operators and term explosion/implosion. This special status makes it possible to adapt the semantics of ListTraversal, treating lists as varyadic constructors instead of as constructed with Nil and Cons.

-- EelcoVisser - 06 Jan 2002

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CategoryToDo^? | ToDo | CompilerImprovements