Jensen's Device: Difference between revisions

== Icon and Unicon ==

Traditional call by name and reference are not features of Icon/Unicon. Procedures parameters are passed by value (immutable types) and reference (mutable types). However, a similar effect may be accomplished by means of co-expressions. The example below was selected for cleanliness of calling.

==={{header|Icon}}===

<lang Icon>record mutable(value) # record wrapper to provide mutable access to immutable types

procedure main()

A := mutable()

write( sum(A, 1, 100, create 1.0/A.value) )

end

procedure sum(A, lo, hi, term)

temp := 0

every A.value := lo to hi do

temp +:= @^term

return temp

end</lang>

Refreshing the co-expression above is more expensive to process but to avoid it requires unary alternation in the call.

<lang Icon> write( sum(A, 1, 100, create |1.0/A.value) )

...

temp +:= @term</lang>

Alternately, we can use a programmer defined control operator (PDCO) approach that passes every argument as a co-expression. Again the refresh co-expression/unary iteration trade-off can be made. The call is cleaner looking but the procedure code is less clear. Additionally all the parameters are passed as individual co-expressions.

<lang Icon> write( sum{A.value, 1, 100, 1.0/A.value} )

...

procedure sum(X)

...

every @X[1] := @X[2] to @X[3] do

temp +:= @^X[4]</lang>

==={{header|Unicon}}===

This Icon solution works in Unicon.