// readTerm returns true if it was able to read a term or if there was an
// error reading a term (the error is in a term.Error value). If the stream
// is empty, it returns false.
func (r *TermReader) readTerm(p priority, i *lex.List, o **lex.List, t *term.Term) bool {
// fmt.Printf("\nreading term\n")
if r.tok(lex.EOF, i, o) {
// fmt.Printf("hit EOF\n")
return false
}
if r.term(p, i, o, t) {
if r.tok(lex.FullStop, *o, o) {
return true
} else {
msg := fmt.Sprintf("expected full stop after `%s` but got `%s`", *t, (*o).Value.Content)
*t = term.NewError(msg, (*o).Value)
return true
}
}
msg := fmt.Sprintf("expected term but got `%s`", i.Value.Content)
*t = term.NewError(msg, i.Value)
return false
}
// parse a single term
func (r *TermReader) term(p priority, i *lex.List, o **lex.List, t *term.Term) bool {
var op, f string
var t0, t1 term.Term
var opP, argP priority
// fmt.Printf("seeking term with %s\n", i.Value.Content)
// prefix operator
if r.prefix(&op, &opP, &argP, i, o) && opP <= p && r.term(argP, *o, o, &t0) {
opT := term.NewCallable(op, t0)
return r.restTerm(opP, p, *o, o, opT, t)
}
// list notation for compound terms §6.3.5
if r.tok('[', i, o) && r.term(999, *o, o, &t0) && r.listItems(*o, o, &t1) {
list := term.NewCallable(".", t0, t1)
return r.restTerm(0, p, *o, o, list, t)
}
if r.tok('[', i, o) && r.tok(']', *o, o) {
list := term.NewAtom("[]")
return r.restTerm(0, p, *o, o, list, t)
}
// parenthesized terms
if r.tok('(', i, o) && r.term(1200, *o, o, &t0) && r.tok(')', *o, o) {
// fmt.Printf("open paren %s close paren\n", t0)
return r.restTerm(0, p, *o, o, t0, t)
}
switch i.Value.Type {
case lex.Int: // integer term §6.3.1.1
n := term.NewInt(i.Value.Content)
*o = i.Next()
return r.restTerm(0, p, *o, o, n, t)
case lex.Float: // float term §6.3.1.1
f := term.NewFloat(i.Value.Content)
*o = i.Next()
return r.restTerm(0, p, *o, o, f, t)
case lex.Atom: // atom term §6.3.1.3
a := term.NewAtomFromLexeme(i.Value.Content)
*o = i.Next()
return r.restTerm(0, p, *o, o, a, t)
case lex.String: // double quated string §6.3.7
cl := term.NewCodeListFromDoubleQuotedString(i.Value.Content)
*o = i.Next()
return r.restTerm(0, p, *o, o, cl, t)
case lex.Variable: // variable term §6.3.2
v := term.NewVar(i.Value.Content)
*o = i.Next()
return r.restTerm(0, p, *o, o, v, t)
case lex.Void: // variable term §6.3.2
v := term.NewVar("_")
*o = i.Next()
return r.restTerm(0, p, *o, o, v, t)
case lex.Comment:
*o = i.Next() // skip the comment
return r.term(p, *o, o, t) // ... and try again
}
// compound term - functional notation §6.3.3
if r.functor(i, o, &f) && r.tok('(', *o, o) {
var args []term.Term
var arg term.Term
for r.term(999, *o, o, &arg) { // 999 priority per §6.3.3.1
args = append(args, arg)
if r.tok(')', *o, o) {
break
}
if r.tok(',', *o, o) {
continue
}
panic("Unexpected content inside compound term arguments")
}
f := term.NewTermFromLexeme(f, args...)
return r.restTerm(0, p, *o, o, f, t)
}
*t = term.NewError("Syntax error", i.Value)
return false
}