parsley
4.4.0
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parsley/parsley/parsley.errors/combinator/ErrorMethods

ErrorMethods

parsley.errors.combinator$.ErrorMethods
final implicit class ErrorMethods[P, +A](p: P)(implicit con: P => Parsley[A])

This class exposes helpful combinators that are specialised for generating more helpful errors messages.

This extension class operates on values that are convertible to parsers. It enables the use of error combinators, which can be used for data validation, error annotation, or immediate failing.

Type parameters

P

the type of base value that this class is used on (the conversion to Parsley) is summoned automatically.

Value parameters

con

a conversion that allows values convertible to parsers to be used.

p

the value that this class is enabling methods on.

Attributes

Constructor

This constructor should not be called manually, it is designed to be used via Scala's implicit resolution.

Version

3.0.0

Source
combinator.scala
Graph
Supertypes
class Object
trait Matchable
class Any

Members list

Grouped members

Error Enrichment Combinators

These combinators add additional information - or refine the existing information within - to an error message that has been generated within the scope of the parser they have been called on. These are a very basic, but effective, way of improving the quality of error messages generated by Parsley.

def ?(item: String): Parsley[A]

This combinator changes the expected component of any errors generated by this parser.

This combinator changes the expected component of any errors generated by this parser.

This is just an alias for the label combinator.

Known as <?> in Haskell.

Attributes

See also

label

Since

3.0.0

Source
combinator.scala
def explain(reason: String): Parsley[A]

This combinator adds a reason to error messages generated by this parser.

This combinator adds a reason to error messages generated by this parser.

When this parser fails having not observably* consumed input, this combinator adds reason to the error message, which should justify why the error occured. Unlike error labels, which may persist if more progress is made having not consumed input, reasons are not carried forward in the error message, and are lost.

*a parser is said to observably consume input when error messages generated by a parser p occur at a deeper offset than p originally started at. While this sounds like it is the same as "having consumed input" for the purposes of backtracking, they are disjoint concepts:

  1. in attempt(p), p can observably consume input even though the wider parser does not consume input due to the attempt.

  2. in amend(p), p can consume input and may not backtrack even though the consumption is not observable in the error message due to the amend.

Value parameters

reason

the reason why a parser failed.

Attributes

Returns

a parser that produces the given reason for failure if it fails.

Since

3.0.0

Source
combinator.scala
def hide: Parsley[A]

This combinator hides the expected component of errors generated by this parser.

This combinator hides the expected component of errors generated by this parser.

When this parser fails having not observably* consumed input, this combinator hides any error labels assigned to the expected item by any label combinators, or indeed the base raw labels produced by the input consuming combinators themselves.

This can be useful, say, for hiding whitespace labels, which are not normally useful information to include in an error message for whitespace insensitive grammars.

*a parser is said to observably consume input when error messages generated by a parser p occur at a deeper offset than p originally started at. While this sounds like it is the same as "having consumed input" for the purposes of backtracking, they are disjoint concepts:

  1. in attempt(p), p can observably consume input even though the wider parser does not consume input due to the attempt.

  2. in amend(p), p can consume input and may not backtrack even though the consumption is not observable in the error message due to the amend.

Attributes

Returns

a parser that does not produce an expected component on failure.

Since

3.0.0

Source
combinator.scala
def label(item: String, items: String*): Parsley[A]

This combinator changes the expected component of any errors generated by this parser.

This combinator changes the expected component of any errors generated by this parser.

When this parser fails having not observably* consumed input, the expected component of the generated error message is set to be the given item.

*a parser is said to observably consume input when error messages generated by a parser p occur at a deeper offset than p originally started at. While this sounds like it is the same as "having consumed input" for the purposes of backtracking, they are disjoint concepts:

  1. in attempt(p), p can observably consume input even though the wider parser does not consume input due to the attempt.

  2. in amend(p), p can consume input and may not backtrack even though the consumption is not observable in the error message due to the amend.

Value parameters

item

the name to give to the expected component of any qualifying errors.

items

any further labels to assign to this parser.

Attributes

Returns

a parser that expects item on failure.

Since

3.0.0

Source
combinator.scala

Filtering Combinators

These combinators perform filtering on a parser, with particular emphasis on generating meaningful error messages if the filtering fails. This is particularly useful for data validation within the parser, as very instructive error messages describing what went wrong can be generated. These combinators often filter using a PartialFunction: this may be because they combine filtering with mapping (in which case, the error message is provided separately), or the function may produce a String. In these cases, the partial function is producing the error messages: if the input to the function is defined, this means that it is invalid and the filtering will fail using the message obtained from the succesful partial function invocation.

def collectMsg[B](msg0: String, msgs: String*)(pf: PartialFunction[A, B]): Parsley[B]

This combinator applies a partial function pf to the result of this parser if its result is defined for pf, failing if it is not.

This combinator applies a partial function pf to the result of this parser if its result is defined for pf, failing if it is not.

First, parse this parser. If it succeeds, test whether its result x is in the domain of the partial function pf. If it is defined for pf, return pf(x). Otherwise, if the result was undefined then fail producing a specialised error message with msg. Equivalent to a guardAgainst (whose msggen ignores its argument) followed by a map.

Value parameters

msg0

the first error message to use if the filtering fails.

msgs

the remaining error messages to use if the filtering fails.

pf

the partial function used to both filter the result of this parser and transform it.

Attributes

Returns

a parser which returns the result of this parser applied to pf, if possible.

See also

collect, which is a basic version of this same combinator with no customised error message.

guardAgainst, which is similar to collectMsg, except it does not transform the data.

Since

3.0.0

Note

when this combinator fails (and not this parser itself), it will generate errors rooted at the start of the parse (as if amend had been used) and the caret will span the entire successful parse of this parser.

implemented in terms of collectWith.

Example

A good example of this combinator in use is for handling overflow in numeric literals.

val integer: Parsley[BigInt] = ...
// this should be amended/entrenched for best results
val int16: Parsley[Short] =
   integer.collectMsg("integer literal should within the range -2^16 to +2^16-1") {
       case x if x >= Short.MinValue
              && x <= Short.MaxValue => x.toShort
   }
Source
combinator.scala
def collectMsg[B](msggen: A => Seq[String])(pf: PartialFunction[A, B]): Parsley[B]

This combinator applies a partial function pf to the result of this parser if its result is defined for pf, failing if it is not.

This combinator applies a partial function pf to the result of this parser if its result is defined for pf, failing if it is not.

First, parse this parser. If it succeeds, test whether its result x is in the domain of the partial function pf. If it is defined for pf, return pf(x). Otherwise, if the result was undefined then fail producing a specialised error message with msggen(x). Equivalent to a guardAgainst followed by a map.

Value parameters

msggen

a function that generates the error messages to use if the filtering fails.

pf

the partial function used to both filter the result of this parser and transform it.

Attributes

Returns

a parser which returns the result of this parser applied to pf, if possible.

See also

collect, which is a basic version of this same combinator with no customised error message.

guardAgainst, which is similar to collectMsg, except it does not transform the data.

Since

4.0.0

Note

when this combinator fails (and not this parser itself), it will generate errors rooted at the start of the parse (as if amend had been used) and the caret will span the entire successful parse of this parser.

implemented in terms of collectWith.

Example

A good example of this combinator in use is for handling overflow in numeric literals.

val integer: Parsley[BigInt] = ...
// this should be amended/entrenched for best results
val int16: Parsley[Short] =
   integer.collectMsg(n => Seq(s"integer literal &#36;n is not within the range -2^16 to +2^16-1")) {
       case x if x >= Short.MinValue
              && x <= Short.MaxValue => x.toShort
   }
Source
combinator.scala
def filterOut(pred: PartialFunction[A, String]): Parsley[A]

This combinator filters the result of this parser using the given partial-predicate, succeeding only when the predicate is undefined.

This combinator filters the result of this parser using the given partial-predicate, succeeding only when the predicate is undefined.

First, parse this parser. If it succeeds then take its result x and test if pred.isDefinedAt(x) is true. If it is false, the parser succeeds, returning x. Otherwise, pred(x) will yield a reason reason and the parser will fail with reason provided to the generated error message à la explain.

This is useful for performing data validation, but where a definitive reason can be given for the failure. In this instance, the rest of the error message is generated as normal, with the expected and unexpected components still given, along with any other generated reasons.

Value parameters

pred

the predicate that is tested against the parser result, which also generates errors.

Attributes

Returns

a parser that returns the result of this parser if it fails the predicate.

See also

filterNot, which is a basic version of this same combinator with no customised reason.

guardAgainst, which is similar to filterOut, except it generates a specialised error as opposed to just a reason.

Since

3.0.0

Note

implemented in terms of filterWith.

when this combinator fails (and not this parser itself), it will generate errors rooted at the start of the parse (as if amend had been used) and the caret will span the entire successful parse of this parser.

Example 

scala> import parsley.character.letter
scala> val keywords = Set("if", "then", "else")
scala> val ident = stringOfSome(letter).filterOut {
   case v if keywords.contains(v) => s"keyword &#36;v cannot be an identifier"
}
scala> ident.parse("hello")
val res0 = Success("hello")
scala> ident.parse("if")
val res1 = Failure(..)
Source
combinator.scala
def guardAgainst(pred: PartialFunction[A, Seq[String]]): Parsley[A]

This combinator filters the result of this parser using the given partial-predicate, succeeding only when the predicate is undefined.

This combinator filters the result of this parser using the given partial-predicate, succeeding only when the predicate is undefined.

First, parse this parser. If it succeeds then take its result x and test of pred.isDefinedAt(x) is true. If it is false, the parser succeeds, returning x. Otherwise pred(x) will yield an error message msg and the parser will fail, producing a specialised error only consisting of the message msg à la fail.

This is useful for performing data validation, but where failure is not tied to the grammar but some other property of the results. For instance, with the identifier example given for filterOut, it is reasonable to suggest that an identifier was expected, and a keyword is not a valid identifier: i.e. these components still make sense. Where guardAgainst shines, however, is in scenarios where the expected alternatives, or the unexpected component itself distract from the cause of the error, or are irrelevant in some way. This might be because guardAgainst is checking some property of the data that is possible to encode in the grammar, but otherwise impractical, either because it is hard to maintain or generates poor error messages for the user.

Value parameters

pred

the predicate that is tested against the parser result, which also generates errors.

Attributes

Returns

a parser that returns the result of this parser if it fails the predicate.

See also

filterNot, which is a basic version of this same combinator with no customised error message.

filterOut, which is similar to guardAgainst, except it generates a reason for failure and not a specialised error.

collectMsg, which is similar to guardAgainst, but can also transform the data on success.

Since

4.0.0

Note

when this combinator fails (and not this parser itself), it will generate errors rooted at the start of the parse (as if amend had been used) and the caret will span the entire successful parse of this parser.

implemented in terms of filterWith.

Example

Suppose we are parsing a data-format for graphs, and a restriction has been placed that ensures that the numeric identifiers of each declared node must be ordered. This has, for whatever reason, been specified as a syntactic property of the data. This is possible to encode using context-sensitive parsing (since each new node can only be parsed according to the previous one), but is fairly difficult and impractical. Instead, when all the declarations have been read, a guardAgainst can be used to prevent mis-ordering:

val node = integer
val nodes = many(node).guardAgainst {
   case ns if ns.nonEmpty
           && ns.zip(ns.tail).exists { case (x, y) => x == y } =>
       val Some((x, _)) = ns.zip(ns.tail).find { case (x, y) => x == y }
       Seq(s"node &#36;x has been declared twice")
   case ns if ns.nonEmpty
           && ns.zip(ns.tail).exists { case (x, y) => x > y } =>
       val Some((x, y)) = ns.zip(ns.tail).find { case (x, y) => x > y }
       Seq(s"nodes &#36;x and &#36;y are declared in the wrong order", "all nodes should be ordered")
}
Source
combinator.scala
def unexpectedWhen(pred: PartialFunction[A, String]): Parsley[A]

This combinator filters the result of this parser using the given partial-predicate, succeeding only when the predicate is undefined.

This combinator filters the result of this parser using the given partial-predicate, succeeding only when the predicate is undefined.

First, parse this parser. If it succeeds then take its result x and test if pred.isDefinedAt(x) is true. If it is false, the parser succeeds, returning x. Otherwise, pred(x) will yield a unexpected label and the parser will fail using unexpected and that label.

This is useful for performing data validation, but where a the failure results in the entire token being unexpected. In this instance, the rest of the error message is generated as normal, with the expected components still given, along with any generated reasons.

Value parameters

pred

the predicate that is tested against the parser result, which also generates errors.

Attributes

Returns

a parser that returns the result of this parser if it fails the predicate.

See also

filterNot, which is a basic version of this same combinator with no unexpected message.

filterOut, which is a variant that produces a reason for failure as opposed to an unexpected message.

guardAgainst, which is similar to unexpectedWhen, except it generates a specialised error instead.

unexpectedWithReasonWhen, which is similar, but also has a reason associated.

Since

3.0.0

Note

when this combinator fails (and not this parser itself), it will generate errors rooted at the start of the parse (as if amend had been used) and the caret will span the entire successful parse of this parser.

implemented in terms of filterWith.

Example 

scala> import parsley.character.letter
scala> val keywords = Set("if", "then", "else")
scala> val ident = stringOfSome(letter).unexpectedWhen {
   case v if keywords.contains(v) => s"keyword &#36;v"
}
scala> ident.parse("hello")
val res0 = Success("hello")
scala> ident.parse("if")
val res1 = Failure(..)
Source
combinator.scala
def unexpectedWithReasonWhen(pred: PartialFunction[A, (String, String)]): Parsley[A]

This combinator filters the result of this parser using the given partial-predicate, succeeding only when the predicate is undefined.

This combinator filters the result of this parser using the given partial-predicate, succeeding only when the predicate is undefined.

First, parse this parser. If it succeeds then take its result x and test if pred.isDefinedAt(x) is true. If it is false, the parser succeeds, returning x. Otherwise, pred(x) will yield a unexpected label and the parser will fail using unexpected and that label as well as a reason.

This is useful for performing data validation, but where a the failure results in the entire token being unexpected. In this instance, the rest of the error message is generated as normal, with the expected components still given, along with any generated reasons.

Value parameters

pred

the predicate that is tested against the parser result, which also generates errors.

Attributes

Returns

a parser that returns the result of this parser if it fails the predicate.

See also

filterNot, which is a basic version of this same combinator with no unexpected message or reason.

filterOut, which is a variant that just produces a reason for failure with no unexpected message.

guardAgainst, which is similar to unexpectedWhen, except it generates a specialised error instead.

unexpectedWhen, which is similar, but with no associated reason.

Since

4.2.0

Note

implemented in terms of filterWith.

Example 

scala> import parsley.character.letter
scala> val keywords = Set("if", "then", "else")
scala> val ident = stringOfSome(letter).unexpectedWhenWithReason {
   case v if keywords.contains(v) => (s"keyword &#36;v", "keywords cannot be identifiers")
}
scala> ident.parse("hello")
val res0 = Success("hello")
scala> ident.parse("if")
val res1 = Failure(..)
Source
combinator.scala

Generic Filtering Combinators

This combinators generalise the combinators from above, which are all special cases of them. Each of these takes the characteristic predicate or function of the regular variants, but takes an errGen object that can be used to fine-tune the error messages. These offer some flexiblity not offered by the specialised filtering combinators, but are a little more verbose to use.

def collectWith[B](errGen: ErrorGen[A])(pf: PartialFunction[A, B]): Parsley[B]

This combinator conditionally transforms the result of this parser with a given partial function, generating an error with the given error generator if the function is not defined on the result of this parser.

This combinator conditionally transforms the result of this parser with a given partial function, generating an error with the given error generator if the function is not defined on the result of this parser.

Like collect, except allows for the error message generated to be fine-tuned with respect to the parsers result and width of input consumed using an ErrorGen object.

Value parameters

errGen

how to generate error messages based on the result of this parser.

pf

the partial function used to both filter the result of this parser and transform it.

Attributes

Since

4.4.0

Source
combinator.scala
def filterWith(errGen: ErrorGen[A])(pred: A => Boolean): Parsley[A]

This combinator filters the result of this parser with the given predicate, generating an error with the given error generator if the function returned false.

This combinator filters the result of this parser with the given predicate, generating an error with the given error generator if the function returned false.

Like filter, except allows for the error message generated to be fine-tuned with respect to the parsers result and width of input consumed using an ErrorGen object.

Value parameters

errGen

how to generate error messages based on the result of this parser.

pred

the predicate that is tested against the parser result.

Attributes

Since

4.4.0

Source
combinator.scala
def mapFilterWith[B](errGen: ErrorGen[A])(f: A => Option[B]): Parsley[B]

This combinator conditionally transforms the result of this parser with a given function, generating an error with the given error generator if the function returns None given the result of this parser.

This combinator conditionally transforms the result of this parser with a given function, generating an error with the given error generator if the function returns None given the result of this parser.

Like mapFilter, except allows for the error message generated to be fine-tuned with respect to the parsers result and width of input consumed using an ErrorGen object.

Value parameters

errGen

how to generate error messages based on the result of this parser.

f

the function used to both filter the result of this parser and transform it.

Attributes

Since

4.4.0

Source
combinator.scala

fail

def !(msggen: A => String): Parsley[Nothing]

This combinator parses this parser and then fails, using the result of this parser to customise the error message.

This combinator parses this parser and then fails, using the result of this parser to customise the error message.

Similar to fail, but first parses this parser: if it succeeded, then its result x is used to form the error message for the fail combinator by calling msggen(x). If this parser fails, however, its error message will be generated instead.

Value parameters

msggen

the generator function for error message, creating a message based on the result of this parser.

Attributes

Returns

a parser that always fails, with the given generator used to produce the error message if this parser succeeded.

Note

this combinator will generate error messages rooted at the start of the previously successful parse of this parser, but only in terms of their position: the actual error is generated at the end of the parse, which means it takes priority over sibling errors. This is because the error concerns the whole parse (for caret) and morally starts where this parser started (as it caused the failure), however, if it had full amend-like behaviour these errors would often disappear.

Deprecated

this combinator has not proven to be particularly useful, and will be replaced by a more appropriate, not exactly the same, verifiedFail combinator.

Source
combinator.scala
def unexpected(msggen: A => String): Parsley[Nothing]

This combinator parses this parser and then fails, using the result of this parser to customise the unexpected component of the error message.

This combinator parses this parser and then fails, using the result of this parser to customise the unexpected component of the error message.

Similar to unexpected, but first parses this parser: if it succeeded, then its result x is used to form the unexpected component of the generated error by calling msggen(x). If this parser fails, however, its error message will be returned untouched.

Value parameters

msggen

the generator function for error message, creating a message based on the result of this parser.

Attributes

Returns

a parser that always fails, with the given generator used to produce an unexpected message if this parser succeeded.

Since

4.2.0

Note

this combinator will generate error messages rooted at the start of the previously successful parse of this parser, but only in terms of their position: the actual error is generated at the end of the parse, which means it takes priority over sibling errors. This is because the error concerns the whole parse (for caret) and morally starts where this parser started (as it caused the failure), however, if it had full amend-like behaviour these errors would often disappear.

Deprecated

this combinator has not proven to be particularly useful and will be removed in 5.0.0. There is a similar, but not exact replacement called verifiedUnexpected.

Source
combinator.scala
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