<!-- fifo.mldoc -->
<!-- Entities.sgml entry
<!ENTITY Fifo SDATA "fifo-sig.sml">
-->
<!DOCTYPE ML-DOC SYSTEM>
<COPYRIGHT OWNER="Bell Labs, Lucent Technologies" YEAR=1998>
<VERSION VERID="1.0" YEAR=1998 MONTH=5 DAY=14>
<TITLE>The Fifo structure</TITLE>
<INTERFACE>
<HEAD>The <CD/Fifo/ structure</HEAD>
<SEEALSO>
<STRREF TOPID/Queue/
</SEEALSO>
<PP>
The <STRREF NOLINK/Fifo/ structure provides an applicative implementation
of queues, in one of the greater abuses of notation.
<PP>
As the implementation uses a pair of lists, the amortized cost for
calling <CD/enqueue/, <CD/dequeue/ or <CD/head/ is constant time.
<STRUCTURE STRID="Fifo">
<SIGBODY SIGID="FIFO" FILE=FIFO>
<SPEC>
<TYPE><TYPARAM>'a<ID>fifo
<SPEC>
<EXN>Dequeue
<SPEC>
<VAL>empty<TY>'a fifo
<COMMENT>
<PROTOTY>
empty
</PROTOTY>
is an empty queue.
<SPEC>
<VAL>isEmpty<TY>'a fifo -> bool
<COMMENT>
<PROTOTY>
isEmpty <ARG/fi/
</PROTOTY>
returns true if <ARG/fi/ is empty.
<SPEC>
<VAL>enqueue<TY>('a fifo * 'a) -> 'a fifo
<COMMENT>
<PROTOTY>
enqueue (<ARG/fi/, <ARG/a/)
</PROTOTY>
creates a new queue by appending <ARG/a/ to the tail of <ARG/fi/.
<SPEC>
<VAL>dequeue<TY>'a fifo -> ('a fifo * 'a)
<RAISES><EXNREF STRID="Fifo"/Dequeue/
<COMMENT>
<PROTOTY>
dequeue <ARG/fi/
</PROTOTY>
returns the head of <ARG/fi/ plus the remainder of <ARG/fi/ after
the head is removed. Raises the exception <EXNREF STRID="Fifo"/Dequeue/
if <ARG/fi/ is empty.
<SPEC>
<VAL>delete<TY>('a fifo * ('a -> bool)) -> 'a fifo
<COMMENT>
<PROTOTY>
delete (<ARG/fi/, <ARG/f/)
</PROTOTY>
creates a new queue by deleting from <ARG/fi/ all elements
satisfying the predicate <ARG/f/. The order of the remaining
elements is preserved.
<SPEC>
<VAL>head<TY>'a fifo -> 'a
<RAISES><EXNREF STRID="Fifo"/Dequeue/
<COMMENT>
<PROTOTY>
head <ARG/fi/
</PROTOTY>
returns the head of <ARG/fi/, without changing <ARG/fi/.
Raises the exception <EXNREF STRID="Fifo"/Dequeue/
if <ARG/fi/ is empty.
<SPEC>
<VAL>peek<TY>'a fifo -> 'a option
<COMMENT>
<PROTOTY>
peek <ARG/fi/
</PROTOTY>
returns the head of <ARG/fi/ if it exists; otherwise, returns
<CONREF STRID="Option" DOCUMENT=SML-BASIS-DOC/NONE/.
<SPEC>
<VAL>length<TY>'a fifo -> int
<COMMENT>
<PROTOTY>
length <ARG/fi/
</PROTOTY>
returns the number of elements in <ARG/fi/. At present, this
is a linear time operation.
<SPEC>
<VAL>contents<TY>'a fifo -> 'a list
<COMMENT>
<PROTOTY>
contents <ARG/fi/
</PROTOTY>
returns the elements in <ARG/fi/ in queue order. This
is a linear time operation.
<SPEC>
<VAL>app<TY>('a -> unit) -> 'a fifo -> unit
<COMMENT>
<PROTOTY>
app <ARG/f/ <ARG/fi/
</PROTOTY>
applies the function <ARG/f/ to the elements in <ARG/fi/ in
queue order. This is equivalent to:
<CODE>
List.app f (contents fi)
</CODE>
<SPEC>
<VAL>map<TY>('a -> 'b) -> 'a fifo -> 'b fifo
<COMMENT>
<PROTOTY>
map <ARG/f/ <ARG/fi/
</PROTOTY>
creates a new queue by mapping the elements in <ARG/fi/ by
<ARG/f/. This is equivalent to:
<CODE>
List.foldl (fn (v,q) => enqueue(q,v)) empty (List.map f (contents fi))
</CODE>
<SPEC>
<VAL>foldl<TY>(('a * 'b) -> 'b) -> 'b -> 'a fifo -> 'b
<COMMENT>
<PROTOTY>
foldl <ARG/f/ <ARG/a/ <ARG/fi/
</PROTOTY>
folds the elements of the queue from the head to the tail.
This is equivalent to:
<CODE>
List.foldl f a (contents fi))
</CODE>
<SPEC>
<VAL>foldr<TY>(('a * 'b) -> 'b) -> 'b -> 'a fifo -> 'b
<COMMENT>
<PROTOTY>
foldr <ARG/f/ <ARG/a/ <ARG/fi/
</PROTOTY>
folds the elements of the queue from the tail to the head.
This is equivalent to:
<CODE>
List.foldr f a (contents fi))
</CODE>
</STRUCTURE>
</INTERFACE>
