Task: S-expressions
C: #include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
#include <string.h>

enum { S_NONE, S_LIST, S_STRING, S_SYMBOL };

typedef struct {
	int type;
	size_t len;
	void *buf;
} s_expr, *expr;

void whine(const char *s)
{
	fprintf(stderr, "parse error before ==>%.10s\n", s);
}

expr parse_string(const char *s, char **e)
{
	expr ex = calloc(sizeof(s_expr), 1);
	char buf[256] = {0};
	int i = 0;

	while (*s) {
		if (i >= 256) {
			fprintf(stderr, "string too long:\n");
			whine(s);
			goto fail;
		}
		switch (*s) {
		case '\\':
			switch (*++s) {
			case '\\':
			case '"':	buf[i++] = *s++;
					continue;

			default:	whine(s);
					goto fail;
			}
		case '"':	goto success;
		default:	buf[i++] = *s++;
		}
	}
fail:
	free(ex);
	return 0;

success:
	*(const char **)e = s + 1;
	ex->type = S_STRING;
	ex->buf = strdup(buf);
	ex->len = strlen(buf);
	return ex;
}

expr parse_symbol(const char *s, char **e)
{
	expr ex = calloc(sizeof(s_expr), 1);
	char buf[256] = {0};
	int i = 0;

	while (*s) {
		if (i >= 256) {
			fprintf(stderr, "symbol too long:\n");
			whine(s);
			goto fail;
		}
		if (isspace(*s)) goto success;
		if (*s == ')' || *s == '(') {
			s--;
			goto success;
		}

		switch (*s) {
		case '\\':
			switch (*++s) {
			case '\\': case '"': case '(': case ')':
					buf[i++] = *s++;
					continue;
			default:	whine(s);
					goto fail;
			}
		case '"':	whine(s);
				goto success;
		default:	buf[i++] = *s++;
		}
	}
fail:
	free(ex);
	return 0;

success:
	*(const char **)e = s + 1;
	ex->type = S_SYMBOL;
	ex->buf = strdup(buf);
	ex->len = strlen(buf);
	return ex;
}

void append(expr list, expr ele)
{
	list->buf = realloc(list->buf, sizeof(expr) * ++list->len);
	((expr*)(list->buf))[list->len - 1] = ele;
}

expr parse_list(const char *s, char **e)
{
	expr ex = calloc(sizeof(s_expr), 1), chld;
	char *next;

	ex->len = 0;

	while (*s) {
		if (isspace(*s)) {
			s++;
			continue;
		}

		switch (*s) {
		case '"':
			chld = parse_string(s+1, &next);
			if (!chld) goto fail;
			append(ex, chld);
			s = next;
			continue;
		case '(':
			chld = parse_list(s+1, &next);
			if (!chld) goto fail;
			append(ex, chld);
			s = next;
			continue;
		case ')':
			goto success;

		default:
			chld = parse_symbol(s, &next);
			if (!chld) goto fail;
			append(ex, chld);
			s = next;
			continue;
		}
	}

fail:
	whine(s);
	free(ex);
	return 0;

success:
	*(const char **)e = s+1;
	ex->type = S_LIST;
	return ex;
}

expr parse_term(const char *s, char **e)
{
	while (*s) {
		if (isspace(*s)) {
			s++;
			continue;
		}
		switch(*s) {
		case '(':
			return parse_list(s+1, e);
		case '"':
			return parse_string(s+1, e);
		default:
			return parse_symbol(s+1, e);
		}
	}
	return 0;
}

void print_expr(expr e, int depth)
{
#define sep() for(i = 0; i < depth; i++) printf("    ")
	int i;
	if (!e) return;


	switch(e->type) {
	case S_LIST:
		sep();
		puts("(");
		for (i = 0; i < e->len; i++)
			print_expr(((expr*)e->buf)[i], depth + 1);
		sep();
		puts(")");
		return;
	case S_SYMBOL:
	case S_STRING:
		sep();
		if (e->type == S_STRING) putchar('"');
		for (i = 0; i < e->len; i++) {
			switch(((char*)e->buf)[i]) {
			case '"':
			case '\\':
				putchar('\\');
				break;
			case ')': case '(':
				if (e->type == S_SYMBOL)
					putchar('\\');
			}

			putchar(((char*)e->buf)[i]);
		}
		if (e->type == S_STRING) putchar('"');
		putchar('\n');
		return;
	}
}

int main()
{
	char *next;
	const char *in = "((data da\\(\\)ta \"quot\\\\ed data\" 123 4.5)\n"
			" (\"data\" (!@# (4.5) \"(mo\\\"re\" \"data)\")))";

	expr x = parse_term(in, &next);

	printf("input is:\n%s\n", in);
	printf("parsed as:\n");
	print_expr(x, 0);
	return 0;
}

Go: package main

import (
    "errors"
    "fmt"
    "reflect"
    "strconv"
    "strings"
    "unicode"
)

var input = `((data "quoted data" 123 4.5)
 (data (!@# (4.5) "(more" "data)")))`

func main() {
    fmt.Println("input:")
    fmt.Println(input)

    s, err := parseSexp(input)
    if err != nil {
        fmt.Println("error:", err)
        return
    }

    fmt.Println("\nparsed:")
    fmt.Println(s)

    fmt.Println("\nrepresentation:")
    s.dump(0)
}


type sexp struct {
    i interface{}
}
type qString string
type list []sexp

func (s sexp) String() string {
    return fmt.Sprintf("%v", s.i)
}

func (q qString) String() string {
    return strconv.Quote(string(q))
}

func (l list) String() string {
    if len(l) == 0 {
        return "()"
    }
    b := fmt.Sprintf("(%v", l[0])
    for _, s := range l[1:] {
        b = fmt.Sprintf("%s %v", b, s)
    }
    return b + ")"
}

















func parseSexp(s string) (sexp, error) {
    s1, rem := ps2(s, -1)
    if err, isErr := s1.i.(error); isErr {
        return sexp{}, err
    }
    if rem > "" {
        return s1, errors.New("Left over text: " + rem)
    }
    return s1, nil
}




func ps2(s0 string, n int) (x sexp, rem string) {
    tok, s1 := gettok(s0)
    switch t := tok.(type) {
    case error:
        return sexp{tok}, s1
    case nil: 
        if n < 0 {
            return sexp{errors.New("blank input string")}, s0
        } else {
            return sexp{errors.New("unmatched (")}, ""
        }
    case byte:
        switch {
        case t == '(':
            x, s1 = ps2(s1, 0) 
            if _, isErr := x.i.(error); isErr {
                return x, s0
            }
        case n < 0:
            return sexp{errors.New("unmatched )")}, ""
        default:
            
            return sexp{make(list, n)}, s1
        }
    default:
        x = sexp{tok} 
    }
    if n < 0 {
        
        return x, s1
    }
    
    l, s1 := ps2(s1, n+1)
    
    
    if _, isErr := l.i.(error); !isErr {
        
        l.i.(list)[n] = x
    }
    return l, s1
}
    






func gettok(s string) (tok interface{}, rem string) {
    s = strings.TrimSpace(s)
    if s == "" {
        return nil, ""
    }
    switch s[0] {
    case '(', ')':
        return s[0], s[1:]
    case '"': 
        if i := strings.Index(s[1:], `"`); i >= 0 {
            return qString(s[1 : i+1]), s[i+2:]
        } 
        return errors.New(`unmatched "`), s
    }
    i := 1
    for i < len(s) && s[i] != '(' && s[i] != ')' && s[i] != '"' &&
        !unicode.IsSpace(rune(s[i])) {
        i++
    }
    if j, err := strconv.Atoi(s[:i]); err == nil {
        return j, s[i:]
    }
    if f, err := strconv.ParseFloat(s[:i], 64); err == nil {
        return f, s[i:]
    }
    return s[:i], s[i:]
}

func (s sexp) dump(i int) {
    fmt.Printf("%*s%v: ", i*3, "", reflect.TypeOf(s.i))
    if l, isList := s.i.(list); isList {
        fmt.Println(len(l), "elements")
        for _, e := range l {
            e.dump(i + 1)
        }
    } else {
        fmt.Println(s.i)
    }
}