String , or ByteString. See also the monomorphic versions parseString and parseByteString. All diagnostic messages emitted over the course of the parse attempt are shown to the user on the console. Returns all diagnostic messages emitted over the course of the parse and the answer if the parse was successful. Fully parse a String to a Result.
Fully parse a ByteString to a Result. Source Contents Index trifecta A modern parser combinator library with convenient diagnostics. Contents Feeding a parser more more input Parsing. Rope a StepFail! Rope ErrInfo StepCont! Documentation newtype Parser a Source The type of a trifecta parser The first four arguments are behavior continuations: epsilon success: the parser has consumed no input and has a result as well as a possible Err; the position and chunk are unchanged see pure epsilon failure: the parser has consumed no input and is failing with the given Err; the position and chunk are unchanged see empty committed success: the parser has consumed input and is yielding the result, set of expected strings that would have permitted this parse to continue, new position, and residual chunk to the continuation.
Constructors StepDone! Rope a Parsing is done and has converted the Rope to a final result StepFail! Produced by Haddock version 2. Fields unparser :: forall r. Monad Parser Source. Instance details Defined in Text. Functor Parser Source. MonadFail Parser Source. Applicative Parser Source. Alternative Parser Source. MonadPlus Parser Source. Those we import from the Nat module, not shown here, but available with Pi-forall's source as well.
We need so few things from Nat here, that we could have defined them ourselves, the commented out Nat data type definition gives a taste of what they'd look like, and indeed: doing so was useful when developing the Trifecta parser, as parsing a single file obviously is a easier than handling module imports. The very first -- haskell -- line was just a means to get some Haskell syntax highlighting in Emacs for Pi-forall programs.
The usual way of interacting with Pi-forall is calling its goFilename function in Main. The result is therefore of limited use for any refactoring efforts only, and this was the reason for developing another white space aware parser.
Automatic conversions between different representations, like the one above for Nat from Succ Succ Zero to 2 in the function two , we don't want either for our refactoring purposes. The fact that our Sample. While goFilename is a convenient way to parse, typecheck and pretty-print our Pi-forall code all in one go, sometimes we want more fine grained control, want to see intermediate results, the abstract syntax tree ie.. The following examples are meant as hints, how to "look under the hood" and discover more details, rather than as a complete description of the system.
Having imported some modules initially:. We can get the result of the Either monad with fromRight' from the either package:. Taking this one step further, we can shorten the output to the first characters:. Given that our Sample. Cheating at the source code of goFilename we can get the abstract syntax tree of our Sample. Except monad, like so:. The result is a list of modules, really: Nat. Normally we'd be interested only in the last module parsed Sample. If we were looking at the contents of just a single self-contained file, without any imports of other modules ie.
We need to provide emptyConstructorNames from Syntax. This time we have to "unpeel the onion" of our monad transformer stack ourselves: the aforementioned StateT keeps track of constructor names, and FreshM from unbound is responsible for names and variable bindings.
Note how, without any further knowledge of Succ , Succ n is just parsed as a function application. This may not be what we want: An admittedly ad-hoc prelude helps recognising Succ as a data constructor rather than as a regular function:.
Nevertheless, we feel much more comfortable now, that we have found a way to make our Trifecta parser behave exactly in Pi-forall 's manner Pi-forall mimicry , and understand what's going on. A few examples may be worth looking at and yes, there is a system to this, it may just not be obvious in the first place, how it works :- :.
We started out reducing Pire 's parser and later Pi-forall 's parser to the bare minimum of functions that would still show the above behaviour or lack thereof in Pire 's case initially , i. Then we added some writing capabilities to Pire 's parser: ran it in the RWS Reader-Writer-State monad, rather than just in the State monad, which allowed us sprinkle some tell me 's in the code, and also we added some increment function calls here and there that would increment the state's counter for fresh variables.
That way we found that one particular choice function within the local function definiton of beforeBinder in expProdOrAnnotOrParens was at the culprit, originally:. We could change Pire 's parsing behaviour by adding some increments and tell me 's it just wouldn't behave like Pi-forall 's parser still , like so:.
Likewise commenting out for example the middle comma choice above and in Pi-forall 's parser had an impact in terms of fresh variables' choice. We were still far from real Pi-forall mimicry, no matter what we tried: where we put our increment function calls etc.
But at least that got us on the right track for understanding what's going on:. It is possible to use Parsec the other way around Parsec inside StateT. We were happy with the parser so far, this is the style of the ' parser , and if we wouldn't want to mimic Pi-forall 's behaviour, our choice of fresh vars seemed more natural.
Trifecta parsers cannot be written in a monad transformer TrifectaT style not that we are aware of: it is not in the documentation, we have never seen a Trifecta parser as a monad transformer wrapping a state, it's always the state wrapping the Trifecta parser. Oh, it starts with , cannot just be a var then, but must be one of the other cases, so try them in this order :.
If we are in the second comma case, we must already have tried the first case and succeeded with term : lookAhead doesn't consume the input, put triggers the state's counter increment as if no backtracking had happened. Likewise: if we are in the third case: we must have looked at the other two case already and found a term each time : thus call lookAhead twice. It was used for testing Pire's parser against Pi-forall's original parser at the time, and will possibly be reintroduced again.
Thus functions to translate between the various syntax representations, like untie have disappeared as well for now. Necessarily these expressions declarations, modules etc. Parser notably. The unit tests have been moved from the src directory to their own tests directory, and are treated as a cabal test-suite utests, use them eg.
Three tests currently: unit tests utests , file tests filetests , and doctests. OK, more docs to come, how to use them, but these should go in the docs section, not the changes section. The individual tests in Tests. Yes, one can use :set -XOverloadedStrings on the ghci cabal repl command prompt, but this only so good: often enough, it's not clear what is meant by "foo" then: the String, the Text?
This t2s conversion one wants to do with the least effort of course, and cheating at the ermine source code as for many things Bound related as well , bitraverse Bitraversable etc comes in handy: once expressed, how to bitraverse the expression tree in general, the String to Text conversion of Expr t a becomes just a bitraversal with T. This will hopefully not be a problem as we need to open binders instantiate lambdas etc , as we navigate the tree.
They will be copied to the src directory, where they will subsequently considered by the doctests. We use the general term cabal repl to mean its newer form cabal new-repl in particular. Modules import PiForall.
PrettyPrint import Control.
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Let us break each one down:. This is the simplest type of Trifecta Bet. The horses MUST finish for you to win. This is not a very forgiving bet, but when you risk high, you can win big. A Trifecta box can also be known as a reverse triactor or a combination tricast. In this bet, you choose the top three horses, but they can finish in any order.
This makes it easier to win, but it comes at a cost. A Trifecta box can cover more than three horses. But the increase in possible combinations increases the cost. With four horses, for example, there are 24 possible combinations. The Trifecta Key box keeps the total cost lower, while allowing you to bet on your key players.
For example, you can bet on the first and third place finishers and choose one or more other horses to fill in the second place spot. A Trifecta wheel is where you pick one horse to finish in a specific position, and back all possible combinations with the rest of the field.
You pay for each possible combination, which depends on how many horses are taking part in the race. You can opt for a part wheel Trifecta, which means you pick one horse to finish first, and then pick a select number of horses that will finish second and third. These bets are very popular, because they are a low cost bet.
Of course, the larger the field, the larger the payout. Trifectas are a form of pari-mutuel wagering which means that payouts are calculated based on the share of a betting pool. A new pool is created for each race. The bigger the pool, the bigger the payout if you win. There are four different methods of betting the Trifecta: straight Trifecta betting, Trifecta wheels, Trifecta key part wheels, and Trifecta boxes.
The latter Trifecta wagering strategy, Trifecta boxes, is the most inefficient yet most popular method of betting the Trifecta, and the main reason for large inefficiencies in the Trifecta wagering pools. The simplest and most overused method of Trifecta betting, Trifecta boxes allow you to play multiple Trifecta combinations on a single ticket.
Trifecta boxes can include as many horses as are in a race. You can box the whole field and play every possible Trifecta combination in a race if you like, although this would be an extremely inefficient method of Trifecta betting and would almost guarantee a loss over time.
To calculate the cost of a Trifecta box, multiply the number of horses you would like to box by that number of horses minus one and by that number minus two, and then multiply it by your bet amount. Preferred by most bettors due to their simplicity, Trifecta boxes are the most inefficient method of betting Trifectas. They do, however, require very little mental work while also offering a chance at cashing a ticket with a good payoff.
The problem is that all combinations in any given Trifecta box do not have an equal chance of winning. This leaves a ton of wasted money in the Trifecta betting pools that are there to be had by smart handicappers who structure their bets properly based on probabilities. Based on your handicapping, the probability of the order of finish being , , is higher than the probability of the order of finish being , , Would it not be more efficient to bet more on the Trifecta combinations that have a higher probability of winning and less on the combinations that have a lower probability of winning?
Slightly more efficient than Trifecta box betting, Trifecta wheel betting does have its place and can be lucrative on occasions when you believe one or more of the favorites can finish out of the money. A Trifecta wheel allows you to play a horse in one position in combination with all the other horses in the field to finish in the remaining two positions.
To calculate the cost of wheeling one horse in one position first, second or third with the remainder of the field to finish in the other two positions, multiply the number of horses in the field minus 1 by the number of horses in the field minus two, and multiply that number by the denomination of your bet.
For example:. Below follows the cost of wheeling one horse in one position with ALL remaining horses in the other two positions, based on field size:. This is especially true with large Trifecta wheels. It is best to play full wheels only when you like a horse who is not the favorite to win, while also having a strong opinion that the first and second favorites will finish out of the money.
But there is an even better way to use wheeling to gain a tremendous advantage over the crowd. Intelligent Trifecta bets should be structured to reflect, in your opinion, the most probable outcome of the race, while also covering as many combinations as possible for the smallest outlay of cash. If one or more of the favorites do finish in the money, the bettor is almost certain to lose money.
This type of Trifecta wheel again assumes that the probabilities of any of the remaining horses finishing in the other two positions are equal, which they are not, and also results in money being wasted on low probability combinations.
Viewed 3k times. Improve this question. Not just a "Parser-Combinator" library? So that should fit the bill. No worries. Show 6 more comments. Active Oldest Votes. Improve this answer. Polyparse should be mentioned as well, since unlike parsec it gives you backtracking by default, and allows you to use commit to cut off choice.
Excellent answer! It's worth noting that the only reason scannerless parsing works at all with parser combinators is the ordered choice operator, which provides for disambiguation albeit a very blunt form. It's not entirely clear how to get this same effect with bottom up parsing. Going to the far end of the spectrum though, a generalized algorithm like GLL: github.
Hi Daniel. GLL does work rather well -- though to be pedantic your GLL combinators aren't quite GLL since they can't compute proper follow sets ; since there is the possibility of using the monad or infinite grammars, and so you can't find all the occurrences of a terminal. The approximation is good enough for practical purposes.
I too have a set of GLL-ish combinators that I need to get around to releasing, once I figure out how to polish them up and integrate them coherently into trifecta. Actually, GLL doesn't require follow sets at all, that's just a constant-time optimization.
This is, in fact, what my GLL combinators fall back on in the case where they can't compute a definitive follow set. In general though, follow sets are only necessary if you have an nullable production, which is a surprisingly rare case. One advantage that a grammar driven parser generator for LL could give you, is automatic error recovery and repair. That is a reason why a generator tool may be desirable over just writing a recursive descent parser by hand using a combinator library.
For example 'ell': cocolab. Add a comment. Norman Ramsey Norman Ramsey k 57 57 gold badges silver badges bronze badges. Kim Stebel Kim Stebel Parser combinator libraries, i. Instead they do inefficient back-tracing. JavaCC has the following disadvantages: conflates the lexer and the parser generation. Raphael Raphael 6, 2 2 gold badges 48 48 silver badges 71 71 bronze badges. And furthermore, Java is not a functional language.
Sorry, did only skim the question. Your point stands, though. Sign up or log in Sign up using Google. Sign up using Facebook. Sign up using Email and Password. Post as a guest Name. Email Required, but never shown.
Source Contents Index trifecta A modern parser combinator library with convenient diagnostics. Contents Feeding a parser more more input Parsing. Rope a StepFail! Rope ErrInfo StepCont! Documentation newtype Parser a Source The type of a trifecta parser The first four arguments are behavior continuations: epsilon success: the parser has consumed no input and has a result as well as a possible Err; the position and chunk are unchanged see pure epsilon failure: the parser has consumed no input and is failing with the given Err; the position and chunk are unchanged see empty committed success: the parser has consumed input and is yielding the result, set of expected strings that would have permitted this parse to continue, new position, and residual chunk to the continuation.
Constructors StepDone! Rope a Parsing is done and has converted the Rope to a final result StepFail! Produced by Haddock version 2. Fields unparser :: forall r. Monad Parser Source. Instance details Defined in Text. Functor Parser Source. MonadFail Parser Source. Applicative Parser Source. Alternative Parser Source. MonadPlus Parser Source. TokenParsing Parser Source. LookAheadParsing Parser Source. CharParsing Parser Source. Parsing Parser Source.
Errable Parser Source. DeltaParsing Parser Source.