Parse.add_listform

STRUCTURE

SYNOPSIS

Adds a “list-form” to the built-in grammar, allowing the parsing of strings such as [a; b; c] and {}.

LIBRARY

Parse

DESCRIPTION

The add_listform function allows the user to augment the HOL parser with rules so that it can turn a string of the form

   <ld> str1 <sep> str2 <sep> ... strn <rd>

into the term

   <cons> t1 (<cons> t2 ... (<cons> tn <nilstr>))

where <ld> is the left delimiter string, <rd> the right delimiter, and <sep> is the separator string from the fields of the record argument to the function. The various stri are strings representing the ti terms. Further, the grammar will also parse <ld> <rd> into <nilstr>.

The pp_element lists passed to this function for the separator, leftdelim and rightdelim fields are interpreted as by the add_rule function. These lists must have exactly one TOK element (this provides the string that will be printed), and other formatting elements such as BreakSpace.

The block_info field is a pair consisting of a “consistency” (PP.CONSISTENT, or PP.INCONSISTENT), and an indentation depth (an integer). The standard value for this field is (PP.INCONSISTENT,0), which will cause lists too long to fit in a single line to print with as many elements on the first line as will fit, and for subsequent elements to appear unindented on subsequent lines. Changing the “consistency” to PP.CONSISTENT would cause lists too long for a single line to print with one element per line. The indentation level number specifies the number of extra spaces to be inserted when a line-break occurs.

In common with the add_rule function, there is no requirement that the cons and nilstr fields be the names of constants; the parser/grammar combination will generate variables with these names if there are no corresponding constants.

The HOL pretty-printer is simultaneously aware of the new rule, and terms of the forms above will print appropriately.

FAILURE

Fails if any of the pp_element lists are ill-formed: if they include TM, BeginFinalBlock, or EndInitialBlock elements, or if do not include exactly one TOK element. Subsequent calls to the term parser may also fail or behave unpredictably if the strings chosen for the various fields above introduce precedence conflicts. For example, it will almost always be impossible to use left and right delimiters that are already present in the grammar, unless they are there as the left and right parts of a closefix.

EXAMPLE

The definition of the “list-form” for lists in the HOL distribution is:

   add_listform {separator = [TOK ";", BreakSpace(1,0)],
                 leftdelim = [TOK "["], rightdelim = [TOK "]"],
                 cons = "CONS", nilstr = "NIL",
                 block_info = (PP.INCONSISTENT, 0)};

while the set syntax is defined similarly:

   add_listform {leftdelim = [TOK "{"], rightdelim = TOK ["}"],
                 separator = [";", BreakSpace(1,0)],
                 cons = "INSERT", nilstr = "EMPTY",
                 block_info = (PP.INCONSISTENT, 0)};

USES

Used to make sequential term structures print and parse more pleasingly.

COMMENTS

As with other parsing functions, there is a temp_add_listform version of this function, which has the same effect on the global grammar, but which does not cause this effect to persist when the current theory is exported.

SEEALSO

add_rule