Live Code: Make a Programming Language from Scratch

Continuing the series where I make a programming language live (sort of), this time implement proper if statements and talk about the dangling else problem, and also come up with an unambiguous solution.

Transcript

The following transcript was automatically generated by an algorithm.

00:00 : hello there this is Toby and welcome to
00:03 : the show where we code a programming
00:06 : language life even though this one won't
00:10 : be live I'm gonna be recording this and
00:12 : then uploading it later okay so today
00:16 : what I want to do is implement a proper
00:19 : if statement we kind of already have a
00:23 : way of using if statements basically we
00:27 : said that the if statement was gonna you
00:31 : not even be a special statement we said
00:33 : there's gonna be an if function which
00:37 : can take the conditional value and then
00:39 : two code blocks and code blocks in this
00:44 : language
00:45 : looks like this right and it's basically
00:48 : gonna be a it's a functional approach to
00:51 : doing if statements no so given that we
00:57 : have this if construct why am i
01:00 : implementing if statements well because
01:03 : one this doesn't look terribly great
01:10 : we don't have an else word in this
01:14 : picture so it's not totally it's not
01:17 : it's a little bit unconventional and
01:20 : also another big reason why I want to
01:23 : implement the if statement properly is
01:25 : because if if you're going to be
01:31 : building a programming language yourself
01:35 : and chances are you might implement an
01:37 : if statement properly so I want to do
01:40 : that on the stream so that I provide an
01:44 : example of doing that so I think what
01:46 : I'm gonna do is in addition to allowing
01:50 : this way of calling an if statement will
01:53 : also have a a sort of native way of
01:58 : doing if statements or maybe we want
02:01 : maybe it will take away the native way
02:03 : so I'm gonna convert this if statement
02:07 : example to a another state
02:12 : I'll call it example if - I am gonna
02:20 : double-check my recording software make
02:25 : sure my mic is working because you never
02:28 : know okay
02:30 : it is it does seem to be working and I
02:32 : am recording okay all right so I'm going
02:35 : to do an if statement that kind of looks
02:39 : like this like that I think that's
02:48 : reasonable we want to close I think this
02:52 : is reasonable
02:53 : so first we'll write a parser for this
02:55 : version of the if statement and then
02:58 : we'll write the generator to go along
03:00 : with it
03:01 : one possibility is where we might have
03:07 : so if stamen is actually a very common
03:09 : example in the textbook in the textbook
03:14 : where this is a place we can you can
03:18 : potentially run into ambiguous grammar
03:22 : territory as in there could potentially
03:26 : two different ways to parse the same
03:29 : thing so I'm gonna check my grammar for
03:32 : that and where the place where usually
03:34 : an ambiguous grammar can come into play
03:37 : yes if you have a nested if statement
03:41 : like this so if you have an else if
03:46 : greater than maybe that greater than 21
03:52 : then else if it's greater than 18 and
03:57 : then an else
04:01 : like you take you can thank you can have
04:06 : a beer and then at 18 is like you can
04:11 : have a root beer maybe
04:15 : and then else you can have a glass of
04:22 : water how about that like you can okay
04:28 : that doesn't interesting even though
04:29 : whatever maybe this is like 1313 you can
04:38 : have a root beer so the questions like
04:44 : because this guy here is a valid if
04:48 : statement I think the ambiguity comes I
04:59 : mean let me look at this I'm sure I
05:02 : understand this case for ambiguity first
05:06 : I think the ambiguity comes when you
05:08 : don't have blocks maybe so if you don't
05:13 : even have blocks then it's unclear
05:16 : whether this if statement is this if
05:21 : statement is should be grouped within
05:23 : the else block of the outer if statement
05:28 : so like whether it should be structured
05:29 : as this or whether it should be parsed
05:39 : instead
05:44 : like instead like this I think is that
06:02 : right
06:02 : is this right yeah if you have an else
06:15 : if construct
06:24 : although either way it works doesn't it
06:29 : hmm luckily
06:57 : what's another way to parse this data is
07:01 : there another way to parse this
07:02 : statement so I'm gonna google it grandma
07:15 : and be ambiguous it might be in the QPD
07:25 : article or something
07:28 : elimination of dangling else I think
07:31 : that's what it is
07:32 : so I mean read this to refresh my memory
07:36 : so if if Aiden ass if be done s 1 if a
07:45 : done if be done oh ok
07:49 : it's actually like this so which I don't
07:53 : have this case so if a then if B
08:06 : then s house you don't know what which
08:11 : of the if statements the else belongs to
08:14 : so in my syntax I guess I would write it
08:21 : like if this 242 then when you are else
08:49 : you still does that make sense so it's
08:58 : not clear whether this else belongs to
09:01 : the first if statement or the second if
09:03 : statement if we had blocks though that
09:07 : would not be an issue at all because if
09:12 : we just force the user to use blocks
09:15 : then that would completely not be an
09:19 : issue at all so that's something which
09:25 : which I actually have been doing anyway
09:28 : in it each of the programming languages
09:31 : I've been making I kind of force users
09:34 : to put in blocks so for that reason I've
09:41 : never concerned myself with this
09:45 : particular problem but I think what I
09:48 : would do here what I want to do here is
09:52 : intentionally hit this hip this dramatic
09:57 : ambiguity to to demonstrate it and and
10:01 : then we'll fix it perhaps by and like
10:04 : manda to making blocks mandatory
10:10 : yeah okay so let's do this okay so first
10:22 : make the let's make the grammar so this
10:25 : is a bit of a mess right now I have a
10:27 : lot of examples that I'm not even in
10:29 : attending to make it properly into the
10:32 : language in any case I'm gonna go to the
10:35 : grammar file which is here and have an
10:39 : if statement so I'm gonna say if
10:43 : statement it is is a one of the
10:48 : statement and define the syntax for it
10:57 : let me look at my lecture I like sir I
11:03 : think this is fine so me and the tree
11:08 : white space after their word F and then
11:11 : we're gonna have an expression do we
11:14 : have an expression yes we do so if we
11:19 : have an expression and then some
11:23 : mandatory whitespace and then another
11:25 : expression so that this this it from
11:32 : here to here that's the mandatory
11:34 : whitespace we might want to allow for
11:37 : multi-line mandatory whitespace dough
11:40 : which at the moment we are not doing
11:44 : that so I'm gonna do is make a new
11:49 : symbol that means multi-line whitespace
12:00 : it should consist of a multi-line
12:05 : myspace character or a multi-line
12:11 : whitespace character and then more
12:13 : multi-line whitespace okay where a
12:16 : multi-line whitespace character is
12:18 : either a mandatory white space or or a
12:28 : new line which let me make sure I have a
12:31 : new line accounted for
12:32 : I do not open oh there it is it's called
12:39 : new line all spelled out okay so I
12:41 : believe with this I can have a
12:45 : multi-line white space which is
12:50 : mandatory yes and then I'm gonna say
12:56 : well if statement is expression and then
13:01 : a multi-line whitespace then another not
13:06 : another expression and then yeah
13:10 : actually yes actually another expression
13:16 : which could potentially be a block
13:18 : expression but it could also just be a
13:20 : regular could it be a statement
13:28 : we have expressions and statements an
13:31 : expression can be a code block and a
13:34 : function call and then we have a
13:37 : statement which could be assignments
13:41 : function closed function definitions in
13:43 : if statements we need if statements to
13:46 : be expressions actually true actually no
14:00 : I'm not gonna do it that way I'm gonna
14:04 : say this is a statement I'm gonna see if
14:11 : there's an expression for the
14:13 : conditional and now we have another
14:15 : statement and then else drag this over
14:22 : here and then we're gonna have an else I
14:31 : guess after this statement
14:40 : after the statement we could have
14:43 : multi-language space and then an else
14:46 : and then it could be another statement I
14:52 : suppose and that's the end of the if
15:00 : thing I think does this work I'm gonna
15:05 : just test this to see if it works I
15:09 : guess I should say that probably well as
15:23 : it Stratos I think it might work
15:34 : I'm this fine simple new line is used
15:37 : that's because there's a token coming
15:39 : from the tokenizer so if I mean to use a
15:43 : token coming from the tokenizer and to
15:47 : prepended with the % I forgot about that
15:53 : okay so now let's see then I try to have
16:01 : this guy parse so let me just have this
16:08 : be the example here so if n is greater
16:12 : than 21
16:13 : then if n is greater than 42 you were
16:18 : old
16:18 : other and in this case it's actually not
16:21 : clear which if the else belongs to and
16:25 : that's the dangling else problems so I
16:28 : think we should meet this problem in a
16:31 : parse this example if to unexpected end
16:38 : of file oh that that's to say they
16:49 : didn't successfully parse the if
16:51 : statement I think so we could use nearly
16:54 : test to test this
17:01 : I think with nearly test I have to give
17:06 : it the name of my grandma file and then
17:11 : test some input and because I have my
17:15 : input in a file I'm gonna take my input
17:18 : yeah he wants me to I could paste in the
17:22 : input that's another way to do it I'm
17:30 : gonna use cat the cat come in to to read
17:40 : out this file and then pipe the input
17:42 : into the nearly test command and now it
17:45 : shows me the pars table so I can
17:49 : hopefully if I can understand what this
17:52 : saying then I can hopefully be bugged
17:55 : this situation so let's see what's going
18:02 : on so at this moment it's in the middle
18:05 : of it is in the middle of parsing this
18:07 : if statement so that's good in fact it
18:11 : has finished parsing this statement the
18:13 : dot is all the way at the end here and
18:18 : which begs the question why did not this
18:23 : succeed so after and it sort of
18:28 : succeeded for this statement it is it
18:31 : succeeded in reading this statement do
18:34 : we need something else to complete a
18:38 : statement because a statement is part of
18:44 : a statement which is part of a program
18:47 : so I would have thought after
18:50 : successfully parsing a statement then it
18:55 : would have successfully first a
19:01 : statements
19:05 : oh wait
19:16 : hmm I don't get this so this this is
19:21 : part of this statement symbol started at
19:26 : chart 16 so let's scroll back to look
19:30 : for chart 16
19:41 : so that where our chart 16
19:44 : so from chart 16 is the line that
19:49 : started this statement so this is the
19:53 : rule that started the rule that
19:56 : ultimately completed the if statement
20:00 : here yeah because we know it's completed
20:03 : because the dot is at the end of the
20:05 : rule okay let's go back to chart 16 so
20:10 : due to that it should have completed the
20:15 : statement and it should have for each
20:19 : [Music]
20:22 : which rule that ends with statement in
20:25 : this in this table it should have moved
20:30 : it moved the dot over however the
20:36 : problem is we don't have any rule in
20:38 : here that has is waiting for a statement
20:44 : for some reason and I don't actually
20:47 : know why
20:56 : so this statement was initiated from 16
21:03 : which is the same rule I mean the same
21:07 : chart as here but where is that oh is
21:12 : here oh okay she oh but the else oh I
21:24 : see okay
21:27 : we're parsing this we wear the Ellis
21:33 : Clause needs to be optional that's the
21:36 : problem we do not allow for the else
21:38 : clause to be optional and so let's go
21:42 : ahead and allow the else clause to be
21:44 : optional so okay so I'm gonna put this
21:49 : whole thing the else clause including
21:52 : the white space before it allowed that
21:56 : to be optional I think I can do it like
21:58 : this
21:58 : so let's reparse the parser that did it
22:05 : and parse it and now this time it
22:09 : completed great okay
22:19 : so I'll do this do it this time using
22:23 : this command line and I guess what I
22:32 : want to do now is do I print out the ast
22:40 : as a file after I do that I am supposed
22:50 : to do that oh I bet it head this part
22:58 : yes the the first tree generates
23:01 : multiple results okay perfect perfect
23:03 : perfect so that that is what I expected
23:07 : the parse tree should generate multiple
23:09 : results and will actually look at
23:14 : actually I think it would be helpful
23:18 : this investigation to whenever it
23:21 : generates multiple results we're going
23:24 : to spit out the JSON files for each of
23:27 : their results so let's go ahead and do
23:30 : that I'm gonna actually do this same
23:34 : code here but for each result
23:42 : of the buzzword that results because
23:47 : there are multiple of them we're gonna
23:54 : grab the ast based on the index and the
24:04 : SD file name is actually going to we're
24:10 : gonna append the index to the end of the
24:13 : file name there and then it will write
24:23 : that out okay so let's do that
24:26 : so now hope that links so now yeah it
24:37 : wrote out both both of the parse trees
24:42 : into ast first so take let's take a look
24:45 : at
24:52 : so look at them left and right
24:58 : okay so the left is version zero and I
25:07 : believe on the right OOP still version
25:12 : zero okay now on the right is burger
25:15 : one okay so go look through them and see
25:20 : where the difference is
25:24 : oh one problem
25:33 : oh one problem I actually toss pause and
25:40 : to go back I forgot to write the ast
25:47 : converter for this statement so so let
25:50 : me go ahead and do that so so I'm gonna
25:54 : write an AST converter that takes these
25:58 : symbols that were matched as an array
26:00 : and return an object I'm gonna call this
26:04 : object its type if statement and it's
26:10 : gonna have a conditional which is that
26:14 : guy which is that index 2 and then it
26:21 : has a consequent consequent block which
26:26 : is at index 0 1 2 3 4 index 4 and then
26:33 : the alternate a could come so ultra naté
26:39 : block of the code put some this is index
26:43 : 4 so this is index 5 however because the
26:50 : meat of what we want is actually within
26:53 : here so this would have been index 0 1 2
26:59 : 3 of index 5 however if I did this it
27:03 : wouldn't work because there's a chance
27:05 : that data index 5 is non-existent so use
27:09 : this guard clause to protect against
27:13 : that situation this says if this is
27:15 : non-existent and don't bother
27:17 : calculating that one and just return
27:20 : undefined but if if data data index 5
27:25 : which is that guy does exist then go in
27:28 : there and grab that guy so that's what
27:33 : it's saying let me regenerate the parser
27:42 : now parse it again again it generated
27:45 : two versions of the file and hopefully
27:47 : this time is actually says if statement
27:49 : yes exactly so now let's look at these
27:53 : two versions of the if statement
27:55 : side-by-side and the first version on
27:59 : the Left says we have a conditional that
28:03 : shouldn't be different that part and
28:08 : then this so on the left-hand side the
28:12 : consequent block has another if
28:15 : statement in it on the right-hand side
28:19 : it also does okay that actually does
28:27 : make sense and then with within the
28:31 : conditional of the nested is if
28:33 : statement so this is the outer if
28:35 : statement this is the inner if statement
28:38 : there's a conditional oh maybe I should
28:42 : look at food
28:54 : and have this code we're gonna have this
29:02 : three panel situation does this work
29:10 : we shall see okay so the conditional on
29:15 : the inner if statement is what greater
29:18 : that is a function call
29:20 : it's whether n is greater than 42 so yep
29:24 : so that makes sense that is also the
29:28 : case for this conditional great greater
29:31 : than and 42 and I think the difference
29:38 : will come in the next part which is that
29:40 : the consequence of the first of the
29:47 : nested if statement in the first version
29:50 : let's see is a function called to print
29:55 : printing out you are old the alternate
29:58 : is null
30:01 : whereas for the second version the
30:04 : consequent is printing out your tool but
30:09 : then the alternate is printing out your
30:12 : still and a dough whereas in the first
30:14 : version
30:15 : eldena is no and then the alternate of
30:18 : the outer if statement is is the
30:22 : printing out your student although so in
30:24 : one in the version in the middle here it
30:28 : interpreted as the dangling else belongs
30:32 : to the outer if statement but in the
30:34 : version on the right here it interpreted
30:38 : as the dangling else belongs to the
30:40 : inner if statement so we don't really
30:43 : want this situation going on and and the
30:50 : way I'm going to resolve it is kind of
30:52 : by forced on but just to require that
30:56 : users use blocks with their if
30:59 : statements so if they're forced to use
31:01 : blocks they have to be explicit about
31:04 : which
31:05 : if statement if statement belongs to
31:08 : right so they either has they either
31:11 : have to do this or they have to do this
31:16 : if we force them to use box so that's
31:19 : what I'm gonna do and and the reason and
31:23 : there there might be some of you that
31:25 : are saying well but now you're requiring
31:27 : people to use blocks and that's not so
31:32 : convenient you know what if they want to
31:34 : write less code I guess a my my attitude
31:43 : towards that at this point is I in my
31:46 : own code when I'm writing JavaScript or
31:50 : typescript or see I always force myself
31:54 : to use blocks anyway because if you
31:56 : don't use blocks there's actually a
32:00 : potential for a very very bad kind of
32:03 : error that can happen in particular if
32:09 : you if you write code like this there is
32:12 : actually a pretty good chance that
32:16 : somebody else will come in after you and
32:19 : try to have his statement right right
32:24 : under your line while believing that
32:27 : this statement also believed belongs to
32:30 : the same block and this kind of error is
32:34 : something that has caused a heart bleed
32:37 : but in the SSL library
32:40 : a few years ago so it is actually a
32:44 : problem and so one of the
32:47 : recommendations that I got when I was
32:49 : reading a blog post by eyes extruder I
32:53 : believe the founder of NPM is that you
32:57 : know you know what guys just just use
32:59 : blocks all the time so since then that's
33:03 : what I've been doing so if I'm using
33:05 : blocks all the time
33:06 : you know maybe it's not too big a deal
33:10 : that you just use blocks all the time
33:12 : and
33:13 : you can see be productive writing code
33:16 : that way while you know well-well
33:21 : writing code that's a little more safe
33:24 : perhaps so anyway that's what I'm gonna
33:27 : do is my program language I'm gonna make
33:30 : blocks mandatory on if statements so and
33:35 : what that means is instead of letting
33:38 : that be just any old statement I'm gonna
33:40 : want this guy to be a code block which I
33:44 : think I have already written
33:46 : yes I've written a symbol a some syntax
33:51 : element code the code block I'm just
33:53 : gonna pop it right in there and then in
33:56 : the else there's also gonna be a code
33:58 : block
33:58 : I think that'll probably do it so long
34:03 : as I do this generate the parser again
34:06 : parse it again and now it's it's cool I
34:11 : think I already changed my code to use a
34:15 : code block yeah so code blocks are
34:17 : mandatory now if you try to not use a
34:20 : code block like that will not parse it
34:24 : just will not parse you're forced to use
34:27 : code blocks and that resolves the issue
34:29 : okay so now I suppose what I want to do
34:35 : is to write the generator for this code
34:37 : so let's go ahead and do that so if I
34:39 : try to generate the code all right
34:42 : generate it one more time if I try to
34:45 : generate the code no DHS generate
34:50 : example if 2 dot X TST it doesn't know
34:56 : how to generate code for an if statement
34:58 : so let's fix that
35:13 : and I guess I have a couple of choices
35:17 : here I could have it generate an if
35:24 : statement in JavaScript or I could have
35:27 : it tonight a function like like in in
35:35 : the format of our previous if statement
35:37 : I'm actually gonna have it generate a
35:39 : statement in JavaScript this is a
35:42 : transpiler so yeah let's just translate
35:46 : this guy to JavaScript and in JavaScript
35:49 : if statement what looks something like
35:53 : if and then parentheses so let me get a
35:57 : conditional generate I'm gonna generate
36:06 : the JavaScript code for the conditional
36:11 : I'm gonna call this long hand farm coded
36:14 : conditional okay so this is a JavaScript
36:19 : form for conditional I'm popping it in
36:22 : this if statement and then I'll have
36:24 : these curly brackets and let me think I
36:36 : have a thing that handles code blocks
36:39 : already
36:43 : however at the moment I am calling
36:49 : generate functions to convert a code
36:52 : block into a function and I think I want
36:55 : a different thing in this case somehow
36:57 : sake generate statement block generally
37:06 : actually I was called it generate blocks
37:10 : yeah generate block given a note that is
37:18 : a code block yeah mm-hmm I think so yeah
37:26 : I think that's good enough
37:29 : and this assumed is just going to assume
37:35 : this is a code block in the code could
37:39 : block weight what is a code block the
37:48 : code block is a note that has type of
37:50 : code block and then it's got statements
37:52 : in it okay so I'm actually gonna steal
37:59 : this code here
38:06 : so this piece of code wants a statement
38:09 : so I grabbed it for for that piece of
38:12 : code here and
38:14 : [Music]
38:15 : yeah actually don't need the return in
38:23 : my scenarios because I'm not generating
38:26 : for a function so I'm just the body is
38:28 : just gonna be that simpler and I'm gonna
38:32 : return do I have this indentation oh
38:37 : this indented body code goes through
38:41 : every line and the body splits it up and
38:45 : put some indentation on it I'm gonna
38:47 : extract this guy out into an indent
38:50 : function and then just call it again
38:59 : just call this guy a spring okay so I'm
39:06 : gonna indent replace this that's just
39:09 : indenting the body now that I have a
39:19 : indented body I'm gonna return it as
39:23 : this block
39:31 : okay so I'm hoping this is gonna work
39:34 : for me generate block for no dot-com
39:41 : sequent
39:56 : that's not enough because I also have an
40:00 : else clause but I want to I want to
40:03 : generate it first to see what would come
40:05 : out return equals that's not right
40:18 : cannot read property map of undefined so
40:25 : Oh statements is somehow undefined so
40:33 : note that consequent did I misspell
40:38 : consequence no this is an if statement
40:46 : it should have a consequent yeah it's an
40:53 : if statement you should have a
40:56 : consequent the consequence should have a
40:58 : statements passing that in here
41:04 : statements but it's saying it doesn't
41:14 : mmm 970 that's this line me practice no
41:19 : doubt then look at what it is it's a
41:25 : code block with statements who
41:33 : the consequence is actually a array of
41:39 : ku blocks I don't think that's right
41:44 : I need I need to fix the parser that's
41:47 : not what I want it's the consequence
41:51 : could be what does a code block return
42:08 : the code block should return an object
42:13 : like this so that doesn't make sense
42:20 : them that doesn't really make sense to
42:30 : me
42:48 : in each consequent it's an array of a
42:51 : code block for some weird reason I'm
42:54 : expecting it to be just be the code
42:56 : block object where else is code block
43:03 : being used okay I'm gonna print it out
43:16 : here inside the parser to try to debug
43:18 : this
43:33 : let's parse the thing in each case a
43:38 : where I printed out the code block I'm
43:40 : getting an array of objects why is that
43:46 : happening that doesn't make sense
43:50 : because in a different place
44:17 : in a place where I have an expression
44:21 : let's say you just make a small example
44:26 : code block dot X so this is a code block
44:48 : so I have to have a new line maybe yes
44:55 : new line is mandatory okay so in this
44:59 : one
45:00 : the value is Oh the value is also oh so
45:14 : it's on array of code blocks so this is
45:16 : unexpected
45:20 : I'm kind of wondering why it was working
45:24 : before now
45:34 : ah it's disco clock without parameters
45:41 : I need a ID here to unbox this because
45:46 : yeah yeah yeah and you did a converter
45:49 : and I can just use ID here yes that's
45:53 : what the problem is probably a bug that
45:57 : was introduced after I did a refactoring
46:01 : so I think this should be fixed now
46:05 : yes it's fixed okay all right so back
46:11 : back to what we were doing which was if
46:17 : example 2 so let's reparse that one
46:27 : example if if 2x okay so we get these
46:34 : code block objects that are actually
46:36 : code block objects now great let me get
46:40 : rid of that tongue so the lock statement
46:42 : from the car sir I was there only for
46:47 : debugging purposes okay so now we have
46:52 : this new ast let me get rid of this zero
46:55 : to two different versions of the ast and
47:00 : now we have the new ast where the
47:05 : consequent is just one code block object
47:08 : so now the generate function should be
47:10 : happy about let's just try to generate
47:14 : this
47:20 : file so it generated example if to dodge
47:27 : a yes and it did this and that looks
47:30 : pretty good
47:31 : except that it doesn't have any else
47:33 : statement so that's this pop DL
47:37 : statement in there there's a extra
47:39 : semicolon that we don't really need
47:43 : that's okay I guess not too big a
47:49 : problem so let's let's grab the if
47:53 : statement I mean the else statement I'll
47:58 : say yeah
48:00 : note that alternate which means there's
48:04 : an else statement so we can just return
48:10 : the if statement I guess if there's no
48:12 : else statement but if there is an out
48:13 : statement then it's generally an else
48:16 : statement
48:17 : so all statements should look like
48:21 : there's an else this to this space
48:26 : before the else and then generate plot
48:29 : in for note that alternate
48:38 : and then we're trying if statement plus
48:44 : else statement I'm gonna check out T
48:52 : comes with a lock I have in here okay
48:58 : now let's look at it we have an else
49:01 : statement now great there's these extra
49:05 : semicolons here but my random too much
49:12 : so I'm just gonna leave them there and I
49:16 : think this food should work yeah
49:22 : so you can actually run everything in
49:24 : one step example if - yes I mean not
49:35 : that yes that X and the whole thing from
49:39 : beginning
49:40 : you're still an adult okay you are still
49:47 : an adult if it is greater than 21 is n
49:58 : greater than 21
49:59 : No
50:06 : you're still a child probably is more
50:09 : apt but if you are greater than 2100 it
50:17 : kind of meant to do this
50:18 : okay so I'll do something like this done
50:21 : to else you are a young man or young
50:28 : adult young adult then if you were
50:35 : greater than forty two-year-old
50:36 : otherwise you're still a child let's see
50:39 : if that works
50:41 : you're a young adult
50:46 : 22:14 you were still a child and then if
50:51 : you're 50 you were old okay that looks
50:56 : good that looks like an if statement for
50:59 : me
50:59 : so if we'd have this if statement then
51:03 : we don't actually need the I have a run
51:09 : time I believe we actually need to
51:12 : implement the if statement as a function
51:15 : anymore so I'm gonna take this guy away
51:18 : and yeah think this will work yes okay
51:24 : so I think that's good that's all I
51:28 : wanted to cover probably took a little
51:30 : longer than I'd expected but that's okay
51:35 : I'm happy that I showed that I was able
51:40 : to demonstrate the dangling else problem
51:42 : if you would like to read more about the
51:45 : dangling else problem you can find this
51:48 : article on Wikipedia and yeah and and I
51:53 : did my solution to to solve the dangling
51:58 : else problem is to not allow it from
52:03 : happening in the first place by
52:06 : requiring users of my program language
52:09 : to use blocks like that and if you do
52:13 : that then you will not have the dangling
52:15 : else problem I don't know if this is
52:20 : this solution is what you prefer but if
52:24 : you can come up with a different way of
52:26 : solving it then that that also works
52:31 : this article will probably talk about
52:38 : how to resolve this problem and my guess
52:40 : is most of the trimming on programming
52:43 : languages that have to deal with this
52:45 : problem they resolve it by just picking
52:48 : one of those two always so that in the
52:55 : semantics of their language they always
52:57 : pick one of those interpretations as the
53:01 : correct one
53:02 : rather than allowing both to be quasi
53:05 : correct okay I think I've been talking
53:09 : for long enough so I'm gonna stop this
53:11 : video and thanks for watching

Live Code: Make a Programming Language from Scratch - Part 7

Transcript