• a string A string is text between quotation marks: e.g., "hello" and " spaces Count ". Strings are built-in primitive types (like numbers and symbols ). The only string functions you'll need for this assignment are string? and string=?, both built-in. We won't use any of the string-processing functions you can find mentioned in help-desk

  The intended difference between strings and symbols is that strings are used when you may want to look at individual characters inside the string. (Indeed, the name refers to "characters that have been strung together".) We won't use this aspect of strings, and we'd actually use symbols except that we'll be working with web pages, that use strings. (If curious, see the beginning scheme language manual in helpdesk, for the string-processing functions.)

  , or
• (cons symbol xlist) You might be wondering, why not represent xexprs as its own struct with two entries? -- a tag, and an xlist. Instead, we piggy-back off the built-in cons structure, to represent xexprs. Good question. It probably should be, but for shorthand reasons isn't. (I.e., using the quote syntax for lists, it's extremely convenient to write tagged xexprs; a call to some make-tagged-expr isn't required.) where the first symbol is called the "tag". We'll call this a "tagged expression".

• - empty, or
• - (cons xexpr xlist)

Do the following:

• Make some further examples of xexprs, for testing our later functions.
• Write xexpr=?, which takes two xexpressions and dtermines whether they're equal. (This wil be helpful when doing your test cases.)
• Write the function is-this-a?, which takes in a symbol and an xexpr, and returns whether the xexpr is a list whose first item is the indicated symbol. (An easy function, but handy.)
• strip-comments: Given an xexpr, return an xexpr w/ any 'comment sub-xexprs removed.
• collect-captions: Given an xexpr, return a list of any sub-exprs with a tag 'caption.
• collect-pictures
• wrap-into-html: xlist --> xexpr
• frame-picture: xexpr --> xexpr. Given a picture (ie, a tagged-xexpr with the tag 'picture), return ...html with that image.
• verify-1-tag: Given a xlist and a symbol, return true if there is exactly one top-level tagged-xexpr in the list, with the indicated tag. This is useful in checking that, say, a picture contains at exactly one 'caption tag. What do we mena by "top-level"? Note that the list might contain, say, a table which in turn contains a 'caption tag; that's okay.
• Other possibilities: trim-all-strings, or contains-string, or contains-tag, or count-tags, or verify-1-tag ...

We will use the teachpack xexpr, which exports the following functions:

• url->xexpr
• file->xexpr
• xexpr->file
• current-date-string