A little combinatory parser in VDM-SL
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.
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