Howto:Add a new binary operator to Nasal

Objective

Demonstrate how to add a new binary operator to Nasal

Example: Perl's spaceship operator: <=>

<=> For details, see: http://en.wikipedia.org/wiki/Spaceship_operator

Semantics

The spaceship operator will return 1, 0, or −1 depending on the value of the left argument relative to the right argument. If the left argument is greater than the right argument, the operator returns 1. If the left argument is less than the right argument, the operator returns −1. If the two arguments are equal, the operator returns 0. If the two arguments cannot be compared (e.g. one of them is NaN), the operator returns undef.

Adding a new token

• Open lex.c
• Add a new "TOK_SPCSHIP" (<=>) to the table of lexemes:

// Static table of recognized lexemes in the language
static const struct Lexeme {
    char* str;
    int   tok;
} LEXEMES[] = {
    {"and", TOK_AND},
    {"or",  TOK_OR},
    {"!",   TOK_NOT},
    {"(", TOK_LPAR},
    {")", TOK_RPAR},
    {"[", TOK_LBRA},
    {"]", TOK_RBRA},
    {"{", TOK_LCURL},
    {"}", TOK_RCURL},
    {"*", TOK_MUL},
    {"+", TOK_PLUS},
    {"-", TOK_MINUS},
    {"/", TOK_DIV},
    {"~", TOK_CAT},
    {":", TOK_COLON},
    {".", TOK_DOT},
    {",", TOK_COMMA},
    {";", TOK_SEMI},
    {"=", TOK_ASSIGN},
    {"<",  TOK_LT},
    {"<=", TOK_LTE},
    {"==", TOK_EQ},
    {"!=", TOK_NEQ},
    {">",  TOK_GT},
    {">=", TOK_GTE},
    {"nil", TOK_NIL},
    {"if",    TOK_IF},
    {"elsif", TOK_ELSIF},
    {"else",  TOK_ELSE},
    {"for",     TOK_FOR},
    {"foreach", TOK_FOREACH},
    {"while",   TOK_WHILE},
    {"return",   TOK_RETURN},
    {"break",    TOK_BREAK},
    {"continue", TOK_CONTINUE},
    {"func", TOK_FUNC},
    {"...", TOK_ELLIPSIS},
    {"?", TOK_QUESTION},
    {"var", TOK_VAR},
    {"+=", TOK_PLUSEQ},
    {"-=", TOK_MINUSEQ},
    {"*=", TOK_MULEQ},
    {"/=", TOK_DIVEQ},
    {"~=", TOK_CATEQ},
    {"<=>", TOK_SPCSHIP},
    {"forindex", TOK_FORINDEX},
};

• Next, open parse.h
• add the TOK_SPCSHIP token:

enum tok {
    TOK_TOP=1, TOK_AND, TOK_OR, TOK_NOT, TOK_LPAR, TOK_RPAR, TOK_LBRA,
    TOK_RBRA, TOK_LCURL, TOK_RCURL, TOK_MUL, TOK_PLUS, TOK_MINUS, TOK_NEG,
    TOK_DIV, TOK_CAT, TOK_COLON, TOK_DOT, TOK_COMMA, TOK_SEMI,
    TOK_ASSIGN, TOK_LT, TOK_LTE, TOK_EQ, TOK_NEQ, TOK_GT, TOK_GTE, TOK_SPCSHIP,
    TOK_IF, TOK_ELSIF, TOK_ELSE, TOK_FOR, TOK_FOREACH, TOK_WHILE,
    TOK_RETURN, TOK_BREAK, TOK_CONTINUE, TOK_FUNC, TOK_SYMBOL,
    TOK_LITERAL, TOK_EMPTY, TOK_NIL, TOK_ELLIPSIS, TOK_QUESTION, TOK_VAR,
    TOK_PLUSEQ, TOK_MINUSEQ, TOK_MULEQ, TOK_DIVEQ, TOK_CATEQ,
    TOK_FORINDEX
};

• Next, open codegen.c and modify genExpr():

// ...
    case TOK_MUL:   genBinOp(OP_MUL,    p, t); break;
    case TOK_PLUS:  genBinOp(OP_PLUS,   p, t); break;
    case TOK_DIV:   genBinOp(OP_DIV,    p, t); break;
    case TOK_CAT:   genBinOp(OP_CAT,    p, t); break;
    case TOK_LT:    genBinOp(OP_LT,     p, t); break;
    case TOK_LTE:   genBinOp(OP_LTE,    p, t); break;
    case TOK_EQ:    genBinOp(OP_EQ,     p, t); break;
    case TOK_NEQ:   genBinOp(OP_NEQ,    p, t); break;
    case TOK_GT:    genBinOp(OP_GT,     p, t); break;
    case TOK_GTE:   genBinOp(OP_GTE,    p, t); break;
    case TOK_SPCSHIP:genBinOP(OP_SPACESHIP, p, t); break;
    case TOK_PLUSEQ:  genEqOp(OP_PLUS, p, t);  break;
    case TOK_MINUSEQ: genEqOp(OP_MINUS, p, t); break;
    case TOK_MULEQ:   genEqOp(OP_MUL, p, t);   break;
    case TOK_DIVEQ:   genEqOp(OP_DIV, p, t);   break;
    case TOK_CATEQ:   genEqOp(OP_CAT, p, t);   break;
//...

• Next, open code.h to add the new "OP_SPACESHIP" operator:

enum {
    OP_NOT, OP_MUL, OP_PLUS, OP_MINUS, OP_DIV, OP_NEG, OP_CAT, OP_LT, OP_LTE,
    OP_GT, OP_GTE, OP_EQ, OP_NEQ, OP_EACH, OP_JMP, OP_JMPLOOP, OP_JIFNOTPOP,
    OP_JIFEND, OP_FCALL, OP_MCALL, OP_RETURN, OP_PUSHCONST, OP_PUSHONE,
    OP_PUSHZERO, OP_PUSHNIL, OP_POP, OP_DUP, OP_XCHG, OP_INSERT, OP_EXTRACT,
    OP_MEMBER, OP_SETMEMBER, OP_LOCAL, OP_SETLOCAL, OP_NEWVEC, OP_VAPPEND,
    OP_NEWHASH, OP_HAPPEND, OP_MARK, OP_UNMARK, OP_BREAK, OP_SETSYM, OP_DUP2,
    OP_INDEX, OP_BREAK2, OP_PUSHEND, OP_JIFTRUE, OP_JIFNOT, OP_FCALLH,
    OP_MCALLH, OP_XCHG2, OP_UNPACK, OP_SLICE, OP_SLICE2, OP_SPACESHIP
};

* Next, open code.c to add a new binary operator to the Nasal VM - see function run():
<syntaxhighlight lang="c">
#define BINOP(expr) do { \
    double l = IS_NUM(STK(2)) ? STK(2).num : numify(ctx, STK(2)); \
    double r = IS_NUM(STK(1)) ? STK(1).num : numify(ctx, STK(1)); \
    SETNUM(STK(2), expr);                                         \
    ctx->opTop--; } while(0)

        case OP_PLUS:  BINOP(l + r);         break;
        case OP_MINUS: BINOP(l - r);         break;
        case OP_MUL:   BINOP(l * r);         break;
        case OP_DIV:   BINOP(l / r);         break;
        case OP_LT:    BINOP(l <  r ? 1 : 0); break;
        case OP_LTE:   BINOP(l <= r ? 1 : 0); break;
        case OP_GT:    BINOP(l >  r ? 1 : 0); break;
        case OP_GTE:   BINOP(l > r ? 1 : 0); break;
	case OP_SPACESHIP: BINOP( l > r ? 1: l<r ?-1:0  ); break;
#undef BINOP

Testing the new code:

 print ( 1 <=> 1 );
 print (10 <=> 0 );
 print ( -3 <=> 4 );