"{ Package: 'stx:goodies/petitparser' }"
PPDelegateParser subclass:#PPExpressionParser
instanceVariableNames:'operators'
classVariableNames:''
poolDictionaries:''
category:'PetitParser-Tools'
!
PPExpressionParser comment:'A PPExpressionParser is a parser to conveniently define an expression grammar with prefix, postfix, and left- and right-associative infix operators.
The following code initializes a parser for arithmetic expressions. First we instantiate an expression parser, a simple parser for expressions in parenthesis and a simple parser for integer numbers.
expression := PPExpressionParser new.
parens := $( asParser token trim , expression , $) asParser token trim
==> [ :nodes | nodes second ].
integer := #digit asParser plus token trim
==> [ :token | token value asInteger ].
Then we define on what term the expression grammar is built on:
expression term: parens / integer.
Finally we define the operator-groups in descending precedence. Note, that the action blocks receive both, the terms and the parsed operator in the order they appear in the parsed input.
expression
group: [ :g |
g prefix: $- asParser token trim do: [ :op :a | a negated ] ];
group: [ :g |
g postfix: ''++'' asParser token trim do: [ :a :op | a + 1 ].
g postfix: ''--'' asParser token trim do: [ :a :op | a - 1 ] ];
group: [ :g |
g right: $^ asParser token trim do: [ :a :op :b | a raisedTo: b ] ];
group: [ :g |
g left: $* asParser token trim do: [ :a :op :b | a * b ].
g left: $/ asParser token trim do: [ :a :op :b | a / b ] ];
group: [ :g |
g left: $+ asParser token trim do: [ :a :op :b | a + b ].
g left: $- asParser token trim do: [ :a :op :b | a - b ] ].
After evaluating the above code the ''expression'' is an efficient parser that evaluates examples like:
expression parse: ''-8++''.
expression parse: ''1+2*3''.
expression parse: ''1*2+3''.
expression parse: ''(1+2)*3''.
expression parse: ''8/4/2''.
expression parse: ''8/(4/2)''.
expression parse: ''2^2^3''.
expression parse: ''(2^2)^3''.
Instance Variables:
operators <Dictionary> The operators defined in the current group.'
!
!PPExpressionParser methodsFor:'private'!
build: aParser left: aChoiceParser
^ (aParser separatedBy: aChoiceParser) foldLeft: [ :a :op :b | op first value: a value: op second value: b ]
!
build: aParser postfix: aChoiceParser
^ aParser , aChoiceParser star map: [ :term :ops | ops inject: term into: [ :result :operator | operator first value: result value: operator second ] ]
!
build: aParser prefix: aChoiceParser
^ aChoiceParser star , aParser map: [ :ops :term | ops reversed inject: term into: [ :result :operator | operator first value: operator second value: result ] ]
!
build: aParser right: aChoiceParser
^ (aParser separatedBy: aChoiceParser) foldRight: [ :a :op :b | op first value: a value: op second value: b ]
!
buildOn: aParser
^ self buildSelectors inject: aParser into: [ :term :selector |
| list |
list := operators at: selector ifAbsent: [ #() ].
list isEmpty
ifTrue: [ term ]
ifFalse: [
self
perform: selector with: term
with: (list size = 1
ifTrue: [ list first first ==> [ :operator | Array with: list first second with: operator ] ]
ifFalse: [
list
inject: PPChoiceParser new
into: [ :choice :each | choice / (each first ==> [ :operator | Array with: each second with: operator ]) ] ]) ] ]
!
buildSelectors
^ #(build:prefix: build:postfix: build:right: build:left:)
!
operator: aSymbol parser: aParser do: aBlock
parser isNil
ifTrue: [ ^ self error: 'You did not specify a term when creating the receiver.' ].
operators isNil
ifTrue: [ ^ self error: 'Use #group: to define precedence groups in descending order.' ].
(operators at: aSymbol ifAbsentPut: [ OrderedCollection new ])
addLast: (Array with: aParser asParser with: aBlock)
! !
!PPExpressionParser methodsFor:'specifying'!
group: aOneArgumentBlock
"Defines a priority group by evaluating aOneArgumentBlock."
operators := Dictionary new.
parser := [
aOneArgumentBlock value: self.
self buildOn: parser ]
ensure: [ operators := nil ]
!
left: aParser do: aThreeArgumentBlock
"Define an operator aParser that is left-associative. Evaluate aThreeArgumentBlock with the first term, the operator, and the second term."
self operator: #build:left: parser: aParser do: aThreeArgumentBlock
!
postfix: aParser do: aTwoArgumentBlock
"Define a postfix operator aParser. Evaluate aTwoArgumentBlock with the term and the operator."
self operator: #build:postfix: parser: aParser do: aTwoArgumentBlock
!
prefix: aParser do: aTwoArgumentBlock
"Define a prefix operator aParser. Evaluate aTwoArgumentBlock with the operator and the term."
self operator: #build:prefix: parser: aParser do: aTwoArgumentBlock
!
right: aParser do: aThreeArgumentBlock
"Define an operator aParser that is right-associative. Evaluate aThreeArgumentBlock with the first term, the operator, and the second term."
self operator: #build:right: parser: aParser do: aThreeArgumentBlock
!
term: aParser
"Defines the initial term aParser of the receiver."
parser isNil
ifTrue: [ parser := aParser ]
ifFalse: [ self error: 'Unable to redefine the term.' ]
! !
!PPExpressionParser class methodsFor:'documentation'!
version_SVN
^ '$Id: PPExpressionParser.st,v 1.2 2012-01-13 11:22:50 cg Exp $'
! !