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1 Mariano Ceccato FBK Fondazione Bruno Kessler The TXL Programming Language (2)

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2 The three phases of TXL Parse Transform Unparse Input textParse tree Transformed parse tree Output text blue fish [words] [word] [words] blue [word][empty] fish [words] [word] marlin [empty] marlin

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3 Anatomy of a TXL program Base grammar Grammar overrides Transformation rules The base grammar defines the lexical forms (tokens or terminals) and the syntactic forms (non-terminals). The optional grammar overrides non-terminal of the base grammar. The ruleset defines the set of transformation rules and functions

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4 Anatomy of a TXL program Base Grammar Grammar overrides Transformation rules Example: Expr grammar include Expr.Grammar redefine expr … | exp([number], [number])) include Expr-exp.Grammar rule main rule one rule two

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5 Specifying Lexical Forms Lexical forms specify how the input is partitionated into tokens. Predefined defaults include identifiers [id] (e.g. ABC, rt789), integer and float [number] (e.g. 123, , 3e22), string [string] (e.g. hi there). The tokens statement gives regular expressions for each class of token in the input language. tokens hexnumber 0[xX][\dABCDEFabcdef]+ end tokens Example:

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6 Specifying lexical Forms (contd) Any single char (not [, ]) not preceded by a \ or # simply represents itself. Single char patterns: ex. \d (digits), \a (alphabetic char). Regular expression operators: [PQR] (any one of), (PQR) (sequence of), P*, P+, P?. tokens name regular expression end tokens Regular expression:

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7 Specifying lexical Forms (contd) The keys specifies that certain identifiers are to be treated as unique special symbols. The compounds specifies char seuqences to be treated as a single terminal. The comments specifies the commenting conventions of the input language. By default comments are ignored by TXL. keys procedure repeat program end keys compounds := >= <= end compounds comments /* */ // end comments

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8 Specifying Syntactic Forms The general form of a non-terminal is: define name alternative1 | alternative2 … | alternativeN end define Where each alternative is any sequence of terminal and non terminal (enclosed in square brackets). The special type [program] describes the structure of the entire input.

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9 Specifying Syntactic Forms (contd) Extended BNF-like sequence notation: [repeat x] sequence of zero or more (X*) [list X] comma-separated list [opt X] optional (zero or one) define statements [repeat statement] end define define statements [statement] | [statement] [statements] end define … are equivalent

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10 Specifying Syntactic Forms (contd) define formalParameters ([list formalParameter+]) | [empty] end define define formalParameter [id] : [type] end define define type int | bool end define key procedure begin end int bool end key define proc procedure [id] [forrmalParameters] begin [body] end end define

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11 Ambiguity TXL resolves ambiguities by choosing the first alternative of each non-terminal that can match the input. define T [number] | ([T]) | + [T] | + + [T] end define ++2 T T T T T ++ 2 Example: T-language

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12 Transformation rules TXL has two kinds of transformation rules, rules and functions, which are distinguished by whether they should transform only one (for functions) or many (for rules) occurrences of their pattern. Rules search their scope for the first istance of their target type matching their pattern, transform it, and then reapply to the entire scope until no more matches are found. Functions do not search, but attempt to match only their entire scope to their pattern, transforming it if it matches.

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13 Rules and function function 2To42 replace [number] 2 by 42 end function rule 2To42 replace [number] 2 by 42 end rule > > > Rules search the pattern!

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14 Searching functions function 2To42 replace * [number] 2 by 42 end function Note: change only * > >

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15 Syntax of rules and functions Simplified and given in TXL. rule [ruleid] [repeat formalArgument] [repeat construct_deconstruct_where] replace [type] [pattern] [repeat construct_deconstruct_where] by [replacement] end rule The same for functions! N.B. If the where-condition is false the rule can not be applied and the result is the input-AST.

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16 Built-in functions rule resolveAdd replace [expr] N1 [number] + N2 [number] by N1 [add N2] end rule function add … end function rule resolveAdd replace [expr] N1 [number] + N2 [number] by N1 [+ N2] end rule … are equivalent!

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17 Built-in functions (contd) rule sort replace [repeat number] N1 [number] N2 [number] Rest [repeat number] where N1 [> N2] by N2 N1 Rest end rule > … >

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18 Recursive functions function fact replace [number] n [number] construct nMinusOne [number] n [- 1] where n [> 1] construct factMinusOne [number] nMinusOne [fact] by n [* factMinusOne] end function

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19 Using rule parameters rule resolveConstants replace [repeat statement] const C [id] = V [expr] RestOfscope [repeat statement] by RestOfScope [replaceByValue C V] end rule rule replaceByValue ConstName [id] Value [expr] replace [primary] ConstName by (Value) end rule Example: Const Pi = 3.14; Area := r*r*Pi; Area := r*r*(3.14);

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20 Exercises Implementing the T-language (page 11). Implementing the Calculator.txl. Adding to the expr-grammar the exponential i.e Exp(x, n). Computing the exponential: - in syntax way: ex. Exp(2, 3) ----> 2*2*2 - in semantic way: by means a recursive function that substitute at Exp(x, n) the correct value.

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21 Homework Implementing a simple version of commands-language where commands can be: - assignments i.e. [id] := [expr]; - declarations i.e. const [id] = [number]; Implementing some transformation rules (page 19) that substitute in the assignments identifiers with related values. Example: Const Pi = 3.14; Area := r*r*Pi; Area := r*r*3.14;

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