{"id":5453,"date":"2018-04-22T13:00:00","date_gmt":"2018-04-22T13:00:00","guid":{"rendered":"http:\/\/writeasync.net\/?p=5453"},"modified":"2018-04-20T03:23:41","modified_gmt":"2018-04-20T03:23:41","slug":"basic-solutions-statements","status":"publish","type":"post","link":"http:\/\/writeasync.net\/?p=5453","title":{"rendered":"BASIC solutions: statements"},"content":{"rendered":"

My last post<\/a> showcased what I thought was a complete GW-BASIC expression parsing solution. Hence, I pushed forward with a new project<\/a> to implement statement parsing.<\/p>\n

This went fine for a bit, but I quickly encountered overly complicated situations<\/a>. Without access to the Sprache<\/a> parser implementations, I was left parsing string values to extract expressions used within statements. This was less than ideal, so I pivoted and merged the separate projects<\/a> back into a single one. Almost immediately<\/a> the job became much simpler.<\/p>\n

Commit<\/a> after<\/a> commit<\/a>, I kept implementing support for more and more statements. It was almost too easy! (In general though expression parsing<\/a> really is the hardest part of a compiler front end<\/a>.)<\/p>\n

Of course, these adventures never proceed without at least one critical issue. Just when I thought I was done, I pointed my glorious new line\/statement parser at adventure.bas<\/a>. It blew up on an input that I thought I had handled but slipped through the cracks, an expression like (X AND Y)=Z<\/code>. I put in a quick fix<\/a> but it was not sufficient to handle the general situation. Basically, due to the way that I implemented AND\/OR expressions — having them in the upper level with separate forks for string and numeric expressions — I kept running into examples beyond the reach of my parser. Seeing that I was on the verge of a major refactor, I added several new tests<\/a>. Then I set out to eliminate the string\/number parser dichotomy and instead lean on semantic analysis<\/a> to ensure valid results.<\/p>\n

I started small by just adding the type information<\/a>. With the groundwork laid, I could then add operator type checking<\/a>. From there, it wasn’t a big leap to finalize the simplified grammar with proper type checking<\/a>.<\/p>\n

With the path now fully unblocked, I could complete the final set of statements needed to parse adventure.bas<\/code>! A finishing touch to add the visitor pattern<\/a>, similar to what I had done for expressions, and it was complete.<\/p>\n

The code can now produce a valid AST<\/a> of a sort for some subset of GW-BASIC programs.<\/p>\n

Here is an example program, followed by a textual output of how my GWParse library interprets it:<\/p>\n

\r\n10 CLS\r\n20 INPUT "WHAT IS YOUR NAME";A$\r\n30 IF A$="" THEN PRINT "TRY AGAIN." : GOTO 20\r\n40 PRINT "HELLO, ";A$;"!"\r\n\r\nLine(10, Cls())\r\nLine(20, Input("WHAT IS YOUR NAME", StrV(A)))\r\nLine(30, If(Eq(StrV(A), StrL("")), Print(StrL("TRY AGAIN."))), Goto(20))\r\nLine(40, Print(StrL("HELLO, "), StrV(A), StrL("!")))\r\n<\/pre>\n
Next time, we will see if I can finally complete the trifecta — C# code generation.<\/p>\n","protected":false},"excerpt":{"rendered":"
My last post showcased what I thought was a complete GW-BASIC expression parsing solution. Hence, I pushed forward with a new project to implement statement parsing. This went fine for a bit, but I quickly encountered overly complicated situations. Without… <\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[91,41],"tags":[],"class_list":["post-5453","post","type-post","status-publish","format-standard","hentry","category-design","category-tdd"],"_links":{"self":[{"href":"http:\/\/writeasync.net\/index.php?rest_route=\/wp\/v2\/posts\/5453","targetHints":{"allow":["GET"]}}],"collection":[{"href":"http:\/\/writeasync.net\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"http:\/\/writeasync.net\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"http:\/\/writeasync.net\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"http:\/\/writeasync.net\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=5453"}],"version-history":[{"count":1,"href":"http:\/\/writeasync.net\/index.php?rest_route=\/wp\/v2\/posts\/5453\/revisions"}],"predecessor-version":[{"id":5454,"href":"http:\/\/writeasync.net\/index.php?rest_route=\/wp\/v2\/posts\/5453\/revisions\/5454"}],"wp:attachment":[{"href":"http:\/\/writeasync.net\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=5453"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"http:\/\/writeasync.net\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=5453"},{"taxonomy":"post_tag","embeddable":true,"href":"http:\/\/writeasync.net\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=5453"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}