7.2 Input/Output Actions

Actions:

write_list(List)

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.
write_list : act(!list(@term))

Example:

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

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

with the next output on the same 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:

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

Other control term we can put in the list argument of write_list 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.

q_(String), where String is string will be displayed with string 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_.

| ?? write_list([uq_('Hello')," there\n",wr_(_895),sp_(2),
              _895,sp_(2),wr_(_678),nl_]).

Hello there
A  A  B

_895 = A : Ty1
_678 = B : Ty2

success

read_term(Term)

Matches Term with the next term denoted by the next sequence of characters typed at the terminal followed by fullstop, return.
read_term :act(??term)

Example:

| ?? read_term(X).

[a,b].
X = [a,b] : term

Note that the variables in the term being read are new variables, unconnected to variables in the query.

| ?? read_term(A).

[A].
A = [B] : list(Ty1)
success

Also note that, for example, if the term being read is a compound term that has no type declaration then the query will raise an exception.

| ?? read_term(A).

f(a).
QuLog exception - exception term: input_term_type_error(read_term, "f(a)")