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4.2 Input/Output



Write the elements of List. If the atom nl_ appears in List it is written as a newline.

You can also use sp_(N) where N is a positive integer to insert N spaces. Strings in List are displayed without the string quotes ".." unless you write them with q_("..."). The quotes are then put around the string.
writeL : (![??term]) ~>>


| ?? writeL(["List of atoms ",[a,b], nl_,
          "Set of nats ", {2,1,4,1}]).

List of atoms [a, b]
Set of nats {1, 2, 4}

with the next output on the next line.

nl_ causes a new line to be output as would the string "\n". In fact any of the C string control characters, such as "\t", "\s" for tab and space respectively, can be put into a string and will have the intended effect unless the string is wrapped inside a q_ term. So we could have written the above query as:

| ?? writeL(["List of atoms ",[a,b],
         "\nSet of nats ", {2,1,4,1}]).

Other control term we can put in the list argument of writeL are:

sp_(n), n positive integer. It will display n spaces.

uq_(Atom), where Atom is an atom that normally needs to be quoted. It will be displayed without the single quotes.

wr_(Var), will not display Var as an underscore followed by a sequence of digits, as is normal, but will give it a name such as A, B, C when displayed and will give subsequent occurence of Var in the list to be output using the given name for Var.

The following query illustrates the the use of uq_ and wr_.

| ?? writeL([uq_('Hello')," there\n",wr_(_895),sp_(2),

Hello there
A  A  B

_895 = A : Ty1
_678 = B : Ty2



The same as writeL but with a trailing newline.


| ??writeLine([s("A list "), [a, b], sp_(2), {2, 1, 4, 1},
               s(" followed by a set.")]).

A list [a, b]  {1, 2, 4} followed by a set.

with the next output being at the beginning of the next line.


Unifies Term with the next term denoted by the next sequence of characters typed at the terminal followed by fullstop, return.
readT :(term?) ~>>


| ?? readT(X).

X = f(A) : term

Note that this read remembers the names of variables (the A above). A consequence of this, given the occurs check in unification, is that the following query fails.

| ?? readT(A).


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