The TXL Programming Language 4

1
The TXL Programming Language (4)

• Filippo Ricca
• ITC-Irst
• Istituto per la ricerca Scientifica e Tecnologica
• ricca_at_itc.it

2
Working with Global Variables

• Global variables are a rich and powerful feature
  that can be used for many distinct purposes,
  including
• - global tables.
• - multiple results from a rule.
• - deep parameters.
• - message-passing communication between
  rules in a rule set (e.g, avoiding interference).

3
Setting Global Table

• Global tables can be set up using an export
  clause before the replace clause in the main rule
  of a program.

Example
function main export Table repeat
table_entry Veggie -gt Cabbage
Fruit -gt Apple
Fruit -gt Orange replace program
P program by P
R1 end function
define table_entry stringlit -gt
stringlit end define
4
Adding Table Entry

• Global tables can be modified by exporting a new
  binding for the table based on the imported
  original binding.

function addTableEntry import Table repeat
table_entry construct newEntry
table_entry Veggie -gt XXX
export Table Table . NewEntry
end function
Example
5
Searching in a Table

• Global tables can be easily queried using
  searching deconstructors.

deconstruct table_entry Table Kind
stringlit -gt Orange
Example

• The binding for Kind will be the stringlit
  Fruit. If no match were to be found, then the
  deconstructor would fail.

6
Avoiding interference between rules
function shiftByOne export Flag id
not_found replace number N
number by N replaceOneByTwo
replaceTwoByThree end function function
replaceOneByTwo replace number
1 export Flag found by
2 end function
We want
1 ---gt 2 ---gt 3 2 ---gt 3
function replaceTwoByThree import Flag id
deconstruct Flag not_found
replace number 2 by
3 end function
7
Remarks

• Transformation of a list of contiguos
  if-statements in a case-statement.
• 1) The rule transformTwoCases is necessary.
• 2) It is not possible create a sequence
  repeat T in this way
• - L elem with L repeat T and
  elem T
• - L L with L repeat T and
  L repeat T
• The Built-in function L . elem (or L .
  L) can be used.

If xgt1 then If xlt1 then case endCase
case endCase case endCase
case endCase
8
Exercises

• Adding to the Expr-language the construct let.
• Extending the Calculator.txl at the
  Let-Expr-language.
• - without global-variables (similar to
  pag. 19 slides 2)
• - using global-variables

let begin x 1 y 2 end xy1.
number
The output of Calculator.txl on the example will
be 4.
9
Homework

• Extending the right part of the
  let-declarations (expression
• instead of number).
• Extending the Calculator.txl at the
  New-Let-Expr-language.
• - without global-variables
• - using global-variables

let begin x 2 y 1x x 3 end
xy.
The output of Calculator.txl on the example will
be 4.