once {rolog}

Invoke a query once

Description

Invoke a query once

Usage

once(
  query = call("member", expression(X), list(quote(a), "b", 3L, 4, TRUE, expression(Y))),
  options = list(portray = FALSE),
  env = globalenv()
)

Arguments

query

an R call. The R call consists of symbols, integers and real numbers, character strings, boolean values, expressions, lists, and other calls. Vectors of booleans, integers, floating point numbers, and strings with length N > 1 are translated to prolog compounds !/N, %/N, #/N and $$/N, respectively. The names can be modified with the options below.

options

This is a list of options controlling translation from and to prolog.

• boolvec (see option rolog.boolvec, default is !) is the name of the prolog compound for vectors of booleans.

• intvec, realvec, charvec define the compound names for vectors of integers, doubles and strings, respectively (defaults are %, # and $$).

• If scalar is TRUE (default), vectors of length 1 are translated to scalar prolog elements. If scalar is FALSE, vectors of length 1 are also translated to compounds.

env

The R environment in which the query is run (default: globalenv()). This is mostly relevant for r_eval/2.

Value

If the query fails, FALSE is returned. If the query succeeds, a (possibly empty) list is returned that includes the bindings required to satisfy the query.

See Also

findall() for querying all solutions

query(), submit(), and clear() for fine-grained control over non-deterministic queries

rolog_options() for options controlling R to prolog translation

Examples

# This query returns FALSE
once(call("member", 1, list(quote(a), quote(b), quote(c))))

# This query returns an empty list meaning yes, it works
once(call("member", 3, list(1, 2, 3)))

# This query returns a list stating that it works if X = 1
once(call("member", 1, list(quote(a), expression(X))))

# The same query using simplified syntax
q = quote(member(1, ""[a, .X]))
once(as.rolog(q))

# This query returns a list stating that X = 1 and Z = expression(Y)
once(call("=", list(expression(X), expression(Y)), list(1, expression(Z))))

# This works for X = [1 | _]; i.e. something like [|](1, expression(_6330))
once(call("member", 1, expression(X)))

# This returns S = '1.0' (scalar)
once(call("format", call("string", expression(S)), "~w", list(1)), options=list(scalar=TRUE))
  
# This returns S = '#(1.0)' (vector), because the 1 is translated to #(1.0). 
# To prevent "~w" from being translated to $$("~w"), it is given as an atom.
once(call("format", call("string", expression(S)), as.symbol("~w"), list(1)), 
  options=list(scalar=FALSE))