.. _language-syntax:

Language syntax
---------------

.. class:: nodoc

> module Codec.Pesto.Parse (
> 	  parse
> 	, test
> 	, Instruction(..)
> 	, Quantity(..)
> 	, Unit
> 	, Object
> 	, Approximately(..)
> 	, Amount(..)
> 	, isResult
> 	, isReference
> 	, isAlternative
> 	, isAnnotation
> 	, isAction
> 	, isDirective
> 	, isUnknown
> 	, spaces1
> 	, notspace
> 	) where
> import Data.Char (isSpace)
> import Data.Ratio ((%))
> import Text.Parsec hiding (parse)
> import Text.ParserCombinators.Parsec.Pos (newPos)
> import Text.ParserCombinators.Parsec.Error (newErrorUnknown)
> import Test.HUnit hiding (test)
>
> import Codec.Pesto.Serialize (serialize)

Pesto parses `UTF-8 <https://tools.ietf.org/html/rfc3629>`_ encoded input data
consisting of space-delimited token. Every character within the Unicode
whitespace class is considered a space.

.. _spaces1:

> stream = ((,) <$> getPosition <*> instruction) `sepEndBy` spaces1
> 	<?> "stream"
> spaces1 = many1 space

The following instructions are supported:

> data Instruction =
> 	  Annotation String
> 	| Ingredient Quantity
> 	| Tool Quantity
> 	| Action String
> 	| Reference Quantity
> 	| Result Quantity
> 	| Alternative Quantity
> 	| Directive String
> 	| Unknown String
> 	deriving (Show, Eq)
>
> instruction =
> 	    try annotation
> 	<|> try ingredient
> 	<|> try tool
> 	<|> try action
> 	<|> try result
> 	<|> try alternative
> 	<|> try reference
> 	<|> try directive
> 	<|> try unknown
> 	<?> "instruction"

The pesto grammar has two instruction types: The first one begins with a
start symbol (``start``) and consumes any character up to and including a
terminating symbol (``end``), which can be escaped with a backslash (``\``).

> betweenEscaped :: Char -> Char -> Parsec String () String
> betweenEscaped start end =
> 	   char start
> 	*> many (try (char '\\' *> char end) <|> satisfy (/= end))
> 	<* char end

Annotations and actions both are of this kind:

> annotation = Annotation <$> betweenEscaped '(' ')'
> action = Action <$> betweenEscaped '[' ']'

Here are examples for both:

> testOpterm = [cmpInstruction "(skinless\nboneless)" (Right (Annotation "skinless\nboneless"))
> 	, cmpInstruction "[stir together]" (Right (Action "stir together"))
> 	, cmpInstruction "[stir\\]together]" (Right (Action "stir]together"))
> 	, cmpInstruction "[stir [together]" (Right (Action "stir [together"))]

The second one starts with one identifying character, ignores the following
whitespace characters, and then consumes a ``Quantity``.

> oparg :: Char -> Parsec String () Instruction -> Parsec String () Instruction
> oparg ident cont = char ident *> spaces *> cont
>
> ingredient = oparg '+' (Ingredient <$> quantity)
> tool = oparg '&' (Tool <$> quantity)
> result = oparg '>' (Result <$> quantity)
> alternative = oparg '|' (Alternative <$> quantity)
> reference = oparg '*' (Reference <$> quantity)

Additionally, there are two special instructions. Directives are similar to the
previous instructions but consume a quoted string (``qstr``).

> directive = oparg '%' (Directive <$> qstr)

Unknown instructions are the fallthrough-case and accept anything. They must
not be discarded at this point. The point of accepting anything is to fail as
late as possible while processing input. This gives the parser a chance to
print helpful messages that provide additional aid to the user, who can then fix
the problem.

> unknown = Unknown <$> many1 notspace

Below are examples for these instructions:

> testOparg = [
> 	  cmpInstruction "+100 g flour"
> 	      (Right (Ingredient (Quantity (Exact (AmountRatio (100%1))) "g" "flour")))
> 	, cmpInstruction "&oven"
> 	      (Right (Tool (strQuantity "oven")))
> 	, cmpInstruction ">dough" (Right (Result (strQuantity "dough")))
> 	, cmpInstruction "|trimmings" (Right (Alternative (strQuantity "trimmings")))
> 	, cmpInstruction "*fish"
> 	      (Right (Reference (strQuantity "fish")))
> 	, cmpInstruction3 "% invalid" (Right (Directive "invalid")) "%invalid"
> 	, cmpInstruction3 "* \t\n 1 _ cheese"
> 	      (Right (Reference (Quantity (Exact (AmountRatio (1%1))) "" "cheese")))
> 	      "*1 _ cheese"
> 	, cmpInstruction3 "!invalid" (Right (Unknown "!invalid")) "!invalid"
> 	]

Qstr
++++

Before introducing quantities we need to have a look at qstr, which is used by
them. A qstr, short for quoted string, can be a string enclosed in double
quotes, a single word or the underscore character that represents the
empty string.

> qstr = try (betweenEscaped '"' '"')
> 	<|> word
> 	<|> char '_' *> return ""
 
A word always starts with a letter, followed by any number of non-space
characters.

> word = (:) <$> letter <*> many notspace
> notspace = satisfy (not . isSpace)

The empty string can be represented by two double quotes or the underscore, but
not the empty string itself.

> testQstr = [
> 	  cmpQstr3 "\"\"" (Right "") "_"
> 	, cmpQstr "_" (Right "")
> 	, cmpQstr "" parseError

Any Unicode character with a General_Category major class L (i.e., a letter, see
`Unicode standard section 4.5
<http://www.unicode.org/versions/Unicode7.0.0/ch04.pdf>`_ for example) is
accepted as first character of a word. That includes german umlauts as well as
greek or arabic script. Numbers, separators, punctuation, and others are not
permitted.

> 	, cmpQstr "water" (Right "water")
> 	, cmpQstr "Äpfel" (Right "Äpfel")
> 	, cmpQstr "τυρί" (Right "τυρί")
> 	, cmpQstr "جبن" (Right "جبن")
> 	, cmpQstr "1sugar" parseError
> 	, cmpQstr "+milk" parseError
> 	, cmpQstr "∀onion" parseError

The remaining letters of a word can be any character, including symbols,
numbers, …

> 	, cmpQstr "rump-roast" (Right "rump-roast")
> 	, cmpQstr "v1negar" (Right "v1negar")
> 	, cmpQstr "mush\"rooms" (Right "mush\"rooms")

…but not spaces.

> 	, cmpQstr " tomatoes" parseError
> 	, cmpQstr "tomatoes " parseError
> 	, cmpQstr "lemon juice" parseError
> 	, cmpQstr "sour\tcream" parseError
> 	, cmpQstr "white\nwine" parseError

If a string contains spaces or starts with a special character, it must be
enclosed in double quotes.

> 	, cmpQstr3 "\"salt\"" (Right "salt") "salt"
> 	, cmpQstr "\"+milk\"" (Right "+milk")
> 	, cmpQstr "\"soy sauce\"" (Right "soy sauce")
> 	, cmpQstr "\"1sugar\"" (Right "1sugar")
> 	, cmpQstr "\"chicken\tbreast\nmeat\"" (Right "chicken\tbreast\nmeat")

Doublequotes within a string can be quoted by prepending a backslash. However,
the usual escape codes like \\n, \\t, … will *not* be expanded.

> 	, cmpQstr "\"vine\"gar\"" parseError
> 	, cmpQstr3 "\"vine\\\"gar\"" (Right "vine\"gar") "vine\"gar"
> 	, cmpQstr "\"oli\\ve oil\"" (Right "oli\\ve oil")
> 	, cmpQstr "\"oli\\\\\"ve oil\"" (Right "oli\\\"ve oil")
> 	, cmpQstr3 "\"sal\\tmon\"" (Right "sal\\tmon") "sal\\tmon"
> 	]

Quantity
++++++++

A ``Quantity`` is a triple of ``Approximately``, ``Unit`` and ``Object`` as parameter.

> data Quantity = Quantity Approximately Unit Object deriving (Show, Eq)

The syntactic construct is overloaded and accepts one to three
arguments. If just one is given, it is assumed to be the ``Object``
and ``Approximately`` and ``Unit`` are empty. Two arguments set
``Approximately`` and ``Unit``, which is convenient when the unit implies
the object (minutes usually refer to the object time, for example).

> quantity = try quantityA <|> quantityB

> quantityA = Quantity
> 	<$> approximately
> 	<*  spaces1
> 	<*> unit
> 	<*> (try (spaces1 *> object) <|> return "")

> quantityB = Quantity
> 	<$> return (Exact (AmountStr ""))
> 	<*> return ""
> 	<*> object

> testQuantityOverloaded = [
> 	  cmpQuantity "oven" (exactQuantity (AmountStr "") "" "oven")
> 	, cmpQuantity "10 min" (exactQuantity (AmountRatio (10%1)) "min" "")
> 	, cmpQuantity "100 g flour" (exactQuantity (AmountRatio (100%1)) "g" "flour")

The first two are equivalent to

> 	, cmpQuantity3 "_ _ oven" (exactQuantity (AmountStr "") "" "oven") "oven"
> 	, cmpQuantity3 "10 min _" (exactQuantity (AmountRatio (10%1)) "min" "") "10 min"

Missing units must not be omitted. The version with underscore should be preferred.

> 	, cmpQuantity3 "1 \"\" meal" (exactQuantity (AmountRatio (1%1)) "" "meal") "1 _ meal"
> 	, cmpQuantity "1 _ meal" (exactQuantity (AmountRatio (1%1)) "" "meal")
> 	]

Units and objects are just strings. However, units should be limited to
`well-known metric units <#well-known-units>`_.

> type Unit = String
> unit = qstr
>
> type Object = String
> object = qstr

``Approximately`` is a wrapper for ranges, that is, two amounts separated by a dash,
approximate amounts, prepended with a tilde, and exact amounts without a modifier.

> data Approximately =
> 	  Range Amount Amount
> 	| Approx Amount
> 	| Exact Amount
> 	deriving (Show, Eq)
> 
> approximately = try range <|> try approx <|> exact
> range = Range <$> amount <*> (char '-' *> amount)
> approx = Approx <$> (char '~' *> amount)
> exact = Exact <$> amount

> testQuantityApprox = [
> 	  cmpQuantity "1-2 _ bananas" (Right (Quantity (Range (AmountRatio (1%1)) (AmountRatio (2%1))) "" "bananas"))
> 	, cmpQuantity "1 - 2 _ bananas" parseError
> 	, cmpQuantity "1- 2 _ bananas" parseError
> 	, cmpQuantity "1 -2 _ bananas" parseError
> 	, cmpQuantity "~2 _ bananas" (Right (Quantity (Approx (AmountRatio (2%1))) "" "bananas"))
> 	, cmpQuantity "~ 2 _ bananas" parseError
> 	]

Amounts are limited to rational numbers and strings. There are no real numbers
by design, and implementations should avoid representing rational numbers as
floating point numbers. They are not required and introduce ugly corner cases when
rounding while converting units, for example.

> data Amount =
> 	  AmountRatio Rational
> 	| AmountStr String
> 	deriving (Show, Eq)
>
> amount = try ratio <|> AmountStr <$> qstr

> testQuantityAmount = [
> 	  cmpQuantity "some _ pepper" (exactQuantity (AmountStr "some") "" "pepper")
> 	, cmpQuantity3 "\"some\"-\"a few\" _ bananas" (Right (Quantity (Range (AmountStr "some") (AmountStr "a few")) "" "bananas")) "some-\"a few\" _ bananas"
> 	, cmpQuantity "~\"the stars in your eyes\" _ bananas" (Right (Quantity (Approx (AmountStr "the stars in your eyes")) "" "bananas"))
> 	]

Rational numbers can be an integral, numerator, and denominator, each separated
by a forward slash, just the numerator and denominator, again separated by a
forward slash, or just a numerator with the default denominator 1 (i.e., ordinary
integral number).

> ratio = let toRatio i num denom = AmountRatio ((i*denom+num)%denom) in
> 	    try (toRatio <$> int <*> (char '/' *> int) <*> (char '/' *> int))
> 	<|> try (toRatio <$> return 0 <*> int <*> (char '/' *> int))
> 	<|> try (toRatio <$> return 0 <*> int <*> return 1)

The following representations are all equal with the first one being
the preferred one:

> testQuantityRatio = [
> 	  cmpQuantity "3 _ bananas" (exactQuantity (AmountRatio (3%1)) "" "bananas")
> 	, cmpQuantity3 "3/1 _ bananas" (exactQuantity (AmountRatio (3%1)) "" "bananas")
> 	               "3 _ bananas"
> 	, cmpQuantity3 "3/0/1 _ bananas" (exactQuantity (AmountRatio (3%1)) "" "bananas")
> 	               "3 _ bananas"

Two numbers are numerator and denominator:

> 	, cmpQuantity "3/5 _ bananas" (exactQuantity (AmountRatio (3%5)) "" "bananas")

Three numbers add an integral part:

> 	, cmpQuantity "3/5/7 _ bananas" (exactQuantity (AmountRatio ((3*7+5)%7)) "" "bananas")
> 	, cmpQuantity3 "10/3 _ bananas" (exactQuantity (AmountRatio (10%3)) "" "bananas")
> 	               "3/1/3 _ bananas"

Rational numbers can be used in ranges and mixed with strings too.

> 	, cmpQuantity "1-\"a few\" _ bananas" (Right (Quantity
> 			(Range (AmountRatio (1%1)) (AmountStr "a few")) "" "bananas"))
> 	, cmpQuantity "1/1/2-2 _ bananas" (Right (Quantity
> 			(Range (AmountRatio (3%2)) (AmountRatio (4%2))) "" "bananas"))
> 	, cmpQuantity "~1/1/2 _ bananas" (Right (Quantity
> 			(Approx (AmountRatio (3%2))) "" "bananas"))
> 	]

Appendix
++++++++

Parser main entry point.

> parse = runParser stream () ""
> int = read <$> many1 digit

Test helpers:

> isLeft (Left _) = True
> isLeft _ = False

A generic parser error:

> parseError = Left (newErrorUnknown (newPos "" 0 0))

Compare output of parser ``f`` for string ``str`` with ``expected``. The
expected result can be a parser error, which matches any actual parse error
(first case).

> cmpParser f str (Left _) = TestCase $ assertBool str $ isLeft $ runParser (f <* eof) () "" str
> cmpParser f str expected = str ~: runParser (f <* eof) () "" str ~?= expected

> cmpParseSerialize f str expectp@(Left _) _ = [cmpParser f str expectp]
> cmpParseSerialize f str expectp@(Right expectpval) expects = [
> 	  cmpParser f str expectp
> 	, serialize expectpval ~?= expects]

Wrap qstr test in AmountStr to aid serialization test

> cmpQstr input expectp = cmpQstr3 input expectp input
> cmpQstr3 input (Left expect) _ = [cmpParser (AmountStr <$> qstr) input (Left expect)]
> cmpQstr3 input (Right expect) expects = cmpParseSerialize (AmountStr <$> qstr) input (Right (AmountStr expect)) expects

> cmpQuantity a b = cmpQuantity3 a b a
> cmpQuantity3 = cmpParseSerialize quantity

> cmpInstruction a b = cmpInstruction3 a b a
> cmpInstruction3 = cmpParseSerialize instruction

> exactQuantity a b c = Right (Quantity (Exact a) b c)
> strQuantity = Quantity (Exact (AmountStr "")) ""

> test = [
> 	  "quantity" ~: testQuantityOverloaded
> 			++ testQuantityApprox
> 			++ testQuantityAmount
> 			++ testQuantityRatio
> 	, "qstr" ~: testQstr
> 	, "oparg" ~: testOparg
> 	, "opterm" ~: testOpterm
> 	]

> isResult (Result _) = True
> isResult _ = False
> isReference (Reference _) = True
> isReference _ = False
> isAlternative (Alternative _) = True
> isAlternative _ = False
> isAnnotation (Annotation _) = True
> isAnnotation _ = False
> isAction (Action _) = True
> isAction _ = False
> isDirective (Directive _) = True
> isDirective _ = False
> isUnknown (Unknown _) = True
> isUnknown _ = False