tomooda / VCParser

A little combinatory parser in VDM-SL

VCParser

A little combinatory parser in VDM-SL

Basics

Here I introduce how to use VCParser by writing a binary number parser.

• To create a parser that accepts only '0' (a character),

zero = takeChar('0')

will take a string (seq of char, in VDM-SL) and returns a PARSED value, for example,

zero("011")

will return

mk_PARSED(mk_TREE(nil, "0"), "11")

which says it parsed the input into a tree of "0" and "110" is remained. If failed,

zero("101")

will result in

mk_PARSED(mk_ERROR("Expected '0'"), "101")

which says it couldn't parse the input and "1001" is remained.

• To create a parser that accepts only "1" (a string),

one = takeString("1")

For example,

one("101")

will return

mk_PARSED(mk_TREE(nil, "1"), "01")

• To create a parser that accepts either 0 or 1 (aka "/"),

bin = either([zero, one])

For example,

mk_(
  bin("101"),
  bin("011")
)

will return

mk_(
  mk_PARSED(mk_TREE(nil, "1"), "01"),
  mk_PARSED(mk_TREE(nil, "0"), "11")
)

• To create a parser that accepts a repeated series of binary digits (aka "*"),

binStar = star(bin)

For example,

mk_(
  binStar("101"),
  binStar("")
)

will return

mk_(
  mk_PARSED(mk_TREE(nil, [mk_TREE(nil, "1"), mk_TREE(nil, "0"), mk_TREE(nil, "1")]), []),
  mk_PARSED(mk_TREE(nil, []), [])
)

Please note that the second value in the result pair is NOT an error.

• To create a parser that accepts a binary digit and then the binStar (aka ","),

binPlus = series([bin, binStar])

For example,

mk_(
  binPlus("1001"),
  binPlus("")
)

will return

mk_(
  mk_PARSED(mk_TREE(nil, [mk_TREE(nil, "1"), mk_TREE(nil, [mk_TREE(nil, "0"), mk_TREE(nil, "1")])]), []), 
  mk_PARSED(mk_ERROR("Unexpected EOF"), [])
)

• To create a parser that optionally accepts "0b" prefix before a series of binary digits (aka "[]"),

binary = series([option(takeString("0b")), binPlus])

For example,

mk_(
  binary("0b10"),
  binary("10"),
  binary("0x10")
)

will return

mk_(
  mk_PARSED(mk_TREE(nil, [mk_TREE(nil, "0b"), mk_TREE(nil, [mk_TREE(nil, "1"), mk_TREE(nil, [mk_TREE(nil, "0")])])]), []),
  mk_PARSED(mk_TREE(nil, [mk_TREE(nil, []), mk_TREE(nil, [mk_TREE(nil, "1"), mk_TREE(nil, [mk_TREE(nil, "0")])])]), []),
  mk_PARSED(mk_TREE(nil, [mk_TREE(nil, []), mk_TREE(nil, [mk_TREE(nil, "0"), mk_TREE(nil, [])])]), "x10")
)

• To create a parser that accepts a binary number trimming blanks around it,

binaryTerm = trimBlanks(binary)

For example,

binaryTerm(" 0b10 1")

will return

mk_PARSED(mk_TREE(nil, [mk_TREE(nil, "0b"), mk_TREE(nil, [mk_TREE(nil, "1"), mk_TREE(nil, [mk_TREE(nil, "0")])])]), "1")

• To obtain a flatten tree from compound parser

flatBinStar = concat(binStar)

For example,

mk_(
  binStar("101"),
  flatBinStar("101")
)

will return

mk_(
  mk_PARSED(mk_TREE(nil, [mk_TREE(nil, "1"), mk_TREE(nil, "0"), mk_TREE(nil, "1")]), []),
  mk_PARSED(mk_TREE(nil, "101"), [])
)

• To change an error message,

bin2 = iferror("Accepts only 0 or 1", bin)

For example,

mk_(
	bin2("101"),
  bin2("210")
)

will return

mk_(
  mk_PARSED(mk_TREE(nil, "1"), "01"), 
  mk_PARSED(mk_ERROR("Accepts only 0 or 1"), "210")
)

• To modify a parsed tree,

translate = transtree(lambda tree:TREE & cases tree:
  mk_TREE(-, "0") -> mk_TREE(nil, "zero"),
  mk_TREE(-, "1") -> mk_TREE(nil, "one") end, bin)

For example,

mk_(
	translate("0"),
  translate("1")
)

will return

mk_(
  mk_PARSED(mk_TREE(nil, "zero"), []),
  mk_PARSED(mk_TREE(nil, "one"), [])
)

There are a bunch of other useful combinators and minimal parsers, such as any, digit, alphabet and so on. Please read the VCParser.vdmsl for details.