Constructing ECLiPSe data

## 3.1 Constructing ECLiPSe data

- ECLiPSe atoms and functors
- Building ECLiPSe terms
- Building atomic ECLiPSe terms
- Building ECLiPSe lists
- Building ECLiPSe structures

### 3.1.1 ECLiPSe atoms and functors

        /* ECLiPSe code */
        S = book("Gulliver's Tales","Swift",hardback,fiction),

In the above structure 'hardback' and 'fiction' are atoms. 'book' is the functor of that structure, and it has an arity (number of arguments) of 4.
Each functor and atom is entered into a dictionary, and is always referred to by its dictionary entry. Two classes, `EC_atom` and `EC_functor` are used to access such dictionary entries.
The 'Name' method applies to both, to get their string form. The 'Arity' method can be used to find out how many arguments a functor has.

        /* C++ code */
        EC_functor book("book",4);
        EC_atom hardback("hardback");

        if (book.Arity()) == 4) .. /* evaluates to true */
        if (book == hardback) ..   /* evaluates to false */
        s = hardback.Name();       /* like s = "hardback"; */

### 3.1.2 Building ECLiPSe terms

The `pword` C data type is used to store ECLiPSe terms. In C++ the `EC_word` data type is used. This is used for any C type as well as for ECLiPSe structures and lists. The size remains fixed in all cases, since large terms are constructed on the ECLiPSe global stack.
The consequences of this are that terms will be garbage collected or moved so terms do not survive the execution of ECLiPSe. In particular, one cannot build such terms asynchronously while ECLiPSe is running, for example this precludes building terms from within a signal handler unless it can make sure that ECLiPSe has yielded when it is running.

### 3.1.3 Building atomic ECLiPSe terms

It is possible to simply cast from a number of simple C++ types to build an `EC_word` In addition, functions exist for creating new variables, and for the nil which terminates ECLiPSe lists. In C++ you can just cast.

        /* making simple terms in C++ */
        EC_word w;
        EC_atom hardback("hardback");
        w = (EC_word) "Swift";
        w = (EC_word) hardback;
        w = (EC_word) 1.002e-7;
        w = (EC_word) 12345;
        w = (EC_word) nil();
        w = (EC_word) newvar();

        /* ECLiPSe equivalent code */
        P1 = "Swift",
        P2 = hardback,
        P3 = 1.002e-7,
        P4 = 12345,
        P5 = [],
        P6 = _,

### 3.1.4 Building ECLiPSe lists

The `list(head,tail)` function builds a list out of two terms. Well formed lists have lists as their tail term and a nil ("[]") at the end, or a variable at the end for difference lists.

        /* making the list [1, "b", 3.0] in C++ */
        EC_word w = list(1, list("b", list(3.0, nil())));

The following example shows how you can write functions to build variable length lists.

    /* function to build a list [n,n+1,n+2,.....,m-1,m] */
    EC_word fromto(int n, int m)
    {
        EC_word tail = nil();
        for(int i = m ; i >= n ; i--)
            tail = list(i,tail);
        return tail;
    }

The list is constructed starting from the end, so at all points during its construction you have a valid term. The interface is designed to make it hard to construct terms with uninitialised sub-terms, which is what you would need if you were to construct the list starting with the first elements.

### 3.1.5 Building ECLiPSe structures

The `term(functor,args..)` function is used to build ECLiPSe structures. A number of different functions each with a different number of arguments is defined so as not to disable C++ casting which would be the case if we defined a function with variable arguments.

        /* making s(1,2,3) in C++ */
        EC_functor s_3("s",3);
        EC_word w = term(s_3,1,2,3);

The above interface is convenient for terms with small fixed arities, for much larger terms an array based interface is provided.

        /* making s(1,2,..,n-1,n) */
        EC_word args[n];
        for(int i=0 ; i<n ; i++)
            args[i] = i+1;
        EC_word w = term(EC_functor("s",n),args);