:3002 Wilf 2017

Transcription

1

2 Name one thing that finds bugs early Making sure that the run or execute method for all tables that search have an error message at the end if the search fails to find anything. Readahead and ScannerReadahead self error: 'Syntax or unexpected char error' Readback and Reduce self error: 'Note to implementor: should not happen'

3 What Else Proves Useful For Debugging Having tokens and trees be able to print themselves. But note x := #ABC. astream << x x := 'ABC'. Puts just ABC into the stream x storestring #ABC Retrieves either OR 'ABC'

4 What Proves Useful For Debugging Having tokens and trees be able to print themselves. In Token printon: astream astream << label storestring; << ': '; << symbol storestring This way, we can tell if the label is a string or a symbol (it matters since #abc = 'abc' is false) So we can tell if it s a string or a symbol When printed as printon: astream tabs: amount part of a tree astream cr. amount timesrepeat: [astream << ' ']. self printon: astream

5 What Proves Useful For Debugging Having tokens and trees be able to print themselves. In Tree printon: astream self printon: astream tabs: 0 printon: astream tabs: amount astream cr. amount timesrepeat: [astream << ' ']. astream << label storestring. children do: [:child child printon: astream tabs: amount + 1]

6 Where It Might Have Helped You You had to initialize the 3 parser stacks with something like empty string tokenstack: Token new label: ''; symbol: nil tablenumberstack: 1 treestack: nil Why? Because a readback table at some point will refer to it as a pair ('' 1). Since all your labels are TYPICALLY stored as symbols, this could have impacted you. We could have had pair (#'' 1) in the readback table instead of pair ('' 1). Should we change the tables? '' is not equal to #''

7 What I Added To Debug Your Code In Parser: for use in the loop that executes the tables At the start of the loop, say self log. Then comment it out when you don t want it anymore log table table := tables at: tablenumber. Transcript cr; << scanner peektoken; << ' Table '; << tablenumber; space; << table; << ' '; << left; << ' '; << right; << ' '. table class = ReadaheadTable iftrue: [Transcript << ' SEARCHING FOR '; << self peek label]. table class = ReadbackTable iftrue: [Transcript << ' SEARCHING FOR '; << (Array with: (tokenstack at: left - 1) label with: (tablenumberstack at: left - 1))]. table class = ReduceTable iftrue: [Transcript space; << table nonterminal; << SEARCHING FOR '; << (tablenumberstack at: left - 1]. table class = ShiftBackTable iftrue: [Transcript << ' SHIFT '; << table shiftamount]. (table class = SemanticTable and: [table action == #buildtree:]) iftrue: [ Transcript space; << table action. table parameters do: [:parameter Transcript space; << parameter]]. This doesn t print everything but it s useful It also beats putting print statements all over the place and having to remove them after.

8 What Else Proves Useful Make sure you use collect:, select:, and collect:when: whenever you can. simpler code Smalltalk uses the operator, for string concatenation and you re used to +. Also, it doesn t allow concatenating with a character (only a string) In String + anobject anobject ischaracter iffalse: [^self, anobject]. ^self, anobject asstring

9 Other Things To pop the stacks, you had use a loop to execute tokenstack removelast tablenumberstack removelast treestack removelast There is a removeindex: index but that s clumsy In OrderedCollection acollection removeindexfrom: left to: right (no looping) removeindexfrom: start to: end end to: start by: -1 do: [:index removeindex: index]

10 When Adding To A Collection If you don t want duplicates, you can use addifabsent: anobject addifidenticalabsent: anobject addallifabsent: acollectionofobjects If you want to retrieve the object there, you can use indexof: anobject 0 if not there indexofidentical: anobject But you can t tell if it did add it or not. addallifidenticalabsent: acollectionofobjects 1 or more if there But then you have to fetch it with collection at: index These can be used when constructing a state successor and asking if it s there already but it s clumsy.

11 Adding More Convenient Add Methods In OrderedCollection addifabsentreturningobject: object index index := self indexof: object. Index > 0 iftrue: [^self at: index]. self add: object. ^object addifabsent: object ifadded: ablock index index := self indexof: object. Index > 0 iftrue: [^self at: index]. self add: object. ablock value. ^object You might also want addifidenticalabsentreturningobject You might also want addallifabsent: collection ifadded: ablock followset addifabsent: element ifadded: [keeplooping := true]

12 What s Useful For Building, -, and & FSMs A state with instance variables builderstates1 builderstate2 Implement an = that addifabsent: will use. = astate self class = astate class iffalse: [^false]. (self builderstates1 includesallidentical: astate builderstates1) iffalse: [^false]. (self builderstates2 includesallidentical: astate builderstates2) iffalse: [^false]. self builderstates1 size = astate builderstates1 size iffalse: [^false]. self builderstates2 size = astate builderstates2 size iffalse: [^false]. ^true

13 When Building FSMs and Accumulating States currentstate := 1. states add: initialstate. [currentstate <= states size] whiletrue: [ state := states at: currentstate. Process state which adds more.. currentstate := currentstate + 1. ] Allowed to grow while you iterate states add: initialstate. states do: [:state Process state which adds more.. ]

14 What if anything might we have forgotten? To execute a semantic action, we needed to know that we can say anobject perform: anaction anobject perform: anaction with: p1 anobject perform: anaction with: p1 with: p2 anobject perform: anaction with: p1 with: p2 with: p3 But more generally, it s anobject perform: anaction witharguments: acollection

15 What Did We Need for Semantic Tables The tables give you the general case... (SemanticTable 115 buildtree: walkoneormore: 64) (SemanticTable 116 indent 65) (SemanticTable 116 fliptreefrom:to: ) My mistake for not having one in the sample grammars; e.g., like #indent #indent [] #fliptreefrom:to: [2 5] But note that you wouldn t see such semantic tables until you could generate the tables yourself. The existing grammar doesn t use one.

16 Might We Have Forgotten About Scanner Grammars They are not quite the same as parser grammars. Why not? Scanner grammars use nostack, read, keep Parser grammars use stack, read, nonode There are NO transitions without attributes There is also something more important See next slide

17 Scanner Grammars Versus Parser Grammars Number -> ' '+. OR Number -> " "+. Attribute -> ('stack' "nostack") For a scanner grammar: there is a Single quoted string (called Characters) Double quotes string (called String) $0 transitions, $1 transitions, $2 transitions, etc When you create FSMs for the right parts, you ll see that they have hundreds of transitions For a parser grammar: there is only a Characters transition, or a String transition

18 Anything Else??

19 A Few Other Collection looping facilities are also Useful acollection1 with: acollection2 do: [:oldstate1 :newstate2... For example, if you duplicate a set of states and you want to duplicate transitions that used to go to old states but now need to go to new states... acollection1 indexeddo: [:index :object... For example, if you need to do something special with the first state; e.g., index = 1

20 You could add a method to facilitate closures closureusing: acollectionreturningblock "self closureusing: [:object The last thing computed by your block must be 'a collection of other objects to add to the closure']" "Note: makes use of = for your objects indirectly via addallifabsent: and returns the resulting collection." self do: [:object self addallifabsent: (acollectionreturningblock value: object)] identityclosureusing: acollectionreturningblock "self closureusing: [:object The last thing computed by your block must be 'a collection of other objects to add to the closure']" "Note: makes use of == for your objects indirectly via addallifidenticalabsent: and returns the resulting collection." self do: [:object self addallifidenticalabsent: (acollectionreturningblock value: object)]