r_term.pl -- Translate a Prolog term into an R expression

This module deals with representing an R expression as a Prolog term. The non-terminal r_expression//2 translates the Prolog term into a string that can be sent to R.

The design is inspired by real from Nicos Angelopoulos.

 r_expression(+Term, -Assignments)//

Grammar that creates an R command from a Prolog term. Terms recognised:

• The atoms true and false are mapped to TRUE and FALSE.
• Other Prolog atoms are mapped to an R name. If required, the name is quoted using backquotes.
• A term +(Atom) is mapped to an R string.
• A Prolog string is mapped to an R string. The server should run in UTF-8 mode for exchange of Unicode data.
• A Prolog number is mapped to an R number.
• A Prolog list is added to Assignments. These are used to create a temporary R variable. The R command translation contains the variable name
• Left$Right is translated as is.
• An array index X[I,...] is translated as is. Empty elements in the index, e.g., the R expression a[,3] can be written as `a['',3]`, `a[-,3] or a[*,3]`.
• Known operators are passed as infix operators. The following operators are known: +, -, *, /, mod, '%%', ^, >=, >, ==, <, <=, =<, \=, '!=', :, <-
• Expr1,Expr2 is translated into two R statements separated by a newline.
• Compound terms are translated to function calls.

This library loads r_expand_dot.pl, which uses the `.` infix operator to make a.b and a.b() valid syntax.

Arguments:

Assignments

- is a list Name=Value for data assignments.

 r_string_codes(+Codes, +Esc)//[private]

Emit an escaped R string. @tbd Do we need to use escape characters?

 r_infix_op(Op, Rop, Priority, Associativity)[private]

True if Op is the Prolog representation for the R operator Rop. The R gammar doesn't specify the ranking of the operators. We use Prolog's rules for now.

 r_prefix_op(Op, Rop, Priority, Associativity)[private]

True if Op is the Prolog representation for the R operator Rop.