How to resolve the algorithm S-expressions step by step in the Potion programming language
How to resolve the algorithm S-expressions step by step in the Potion programming language
Table of Contents
Problem Statement
S-Expressions are one convenient way to parse and store data.
Write a simple reader and writer for S-Expressions that handles quoted and unquoted strings, integers and floats. The reader should read a single but nested S-Expression from a string and store it in a suitable datastructure (list, array, etc). Newlines and other whitespace may be ignored unless contained within a quoted string. “()” inside quoted strings are not interpreted, but treated as part of the string. Handling escaped quotes inside a string is optional; thus “(foo"bar)” maybe treated as a string “foo"bar”, or as an error. For this, the reader need not recognize “\” for escaping, but should, in addition, recognize numbers if the language has appropriate datatypes. Languages that support it may treat unquoted strings as symbols. Note that with the exception of “()"” (“\” if escaping is supported) and whitespace there are no special characters. Anything else is allowed without quotes. The reader should be able to read the following input and turn it into a native datastructure. (see the Pike, Python and Ruby implementations for examples of native data structures.) The writer should be able to take the produced list and turn it into a new S-Expression. Strings that don't contain whitespace or parentheses () don't need to be quoted in the resulting S-Expression, but as a simplification, any string may be quoted.
Let the writer produce pretty printed output with indenting and line-breaks.
Let's start with the solution:
Step by Step solution about How to resolve the algorithm S-expressions step by step in the Potion programming language
Source code in the potion programming language
isdigit = (c): 47 < c ord and c ord < 58.
iswhitespace = (c): c ord == 10 or c ord == 13 or c == " ".
# str: a string of the form "...[{}]..."
# i: index to start at (must be the index of )
# => returns (, )
parsesymbol = (str, i) :
datum = ()
while (str(i) != "(" and str(i) != ")" and not iswhitespace(str(i)) and str(i) != "\"") :
datum append(str(i++))
.
(datum join, i)
.
# str: a string of the form "...[]{}[{}]..."
# i: index to start at (must be the index of the first token)
# => returns (, )
parsenumber = (str, i) :
datum = ()
dot = false
while (str(i) != "(" and str(i) != ")" and not iswhitespace(str(i)) and str(i) != "\"") :
if (str(i) == "."): dot = true.
datum append(str(i++))
.
if (dot): (datum join number, i).
else: (datum join number integer, i).
.
# str: a string of the form "...\"....\"..."
# i: index to start at (must be the index of the first quote)
# => returns (, )
parsestring = (str, i) :
datum = ("\"")
while (str(++i) != "\"") :
datum append(str(i))
.
datum append("\"")
(datum join, ++i)
.
# str: a string of the form "...(...)..."
# i: index to start at
# => returns (, )
parselist = (str, i) :
lst = ()
data = ()
while (str(i) != "("): i++.
i++
while (str(i) != ")") :
if (not iswhitespace(str(i))) :
if (isdigit(str(i)) or (str(i) == "-" and isdigit(str(i + 1)))): data = parsenumber(str, i).
elsif (str(i) == "\""): data = parsestring(str, i).
elsif (str(i) == "("): data = parselist(str, i).
else: data = parsesymbol(str, i).
lst append(data(0))
i = data(1)
. else :
++i
.
.
(lst, ++i)
.
parsesexpr = (str) :
parselist(str, 0)(0)
.
parsesexpr("(define (factorial x) \"compute factorial\" (version 2.0) (apply * (range 1 x)))") string print
"\n" print
parsesexpr("((data \"quoted data\" 123 4.5)
(data (!@# (4.5) \"(more\" \"data)\")))") string print
"\n" print
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