02/03/2007 The TXL Programming Language (3)

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The TXL Programming Language (3)

Mariano Ceccato

ITC-Irst

Istituto per la ricerca Scientifica e Tecnologica

ceccato@itc.it

02/03/2007 The TXL Programming Language (3) 2

Rules and functions

function 2To42 replace [number] 2 by 42end function

rule 2To42 replace [number] 2 by 42end rule

2 ----> 4232 6 2 78 4 2

2 ----> 4232 6 2 78 4 2 ----> 42 6 42 78 4 42

Rules search the pattern!Rules search the pattern!

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Searching functions

function 2To42 replace * [number] 2 by 42end function

Note: change only *

2 ----> 4232 6 2 78 4 2 ----> 42 6 2 78 4 2

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Deconstruct and searching functions

rule vectorizeScalarAssignments replace [repeat statement] C1 [statement] C2 [statement] rest [repeat statement] deconstruct C1 V1 [var] := E1 [expr]; deconstruct C2 V2 [var] := E2 [expr]; where not E2 [reference V1] where not E1 [reference V2] construct Passign [statement] <V1, V2> := <E1, E2>; by Passign restend rule

function reference V [variable] match * [variable] Vend function

Example:

x:=x+1;y:=t+4; <x, y>:= <x+1, t+4>;

x:=x+1;y:=x+4; No!

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Extreme programming and TXL

Txl is ideally suited to extreme programming (XP). Begin with an explicit set of testcases, and treat these as the

specification of your transformation. Program your transformation incrementally, as a sequence

of successive approximations to the final result. Actually run your partial transforms against the testcases to

keep track of your progress and test as you go. Always write the simplest possible transformation rules

to achieve the result. Begin each rule with an explicit example pattern and

replacement, and generalize from there.

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Example: XP and TXL (1)

Step 1 – start with an explicit concrete example case.

rule convertAddIJK replace [statement] add I to J giving K % COBOL by K = I + J; % PL/Iend rule

Step 2 – generalize by introducing pattern variables.

rule convertAddGiving replace [statement] add I [Var] to J [Var] giving K [Var] by K = I + J;end rule

N.B. Test at every stage

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Example: XP and TXL (2)

Step 3 – Add new cases.

rule convertAddNoGiving replace [statement] add I [Var] to J [Var] by J = I + J;end rule

Step 4 – Integrate different cases.

rule convertAdds replace [statement] AddStatement [cobol_add_statement] by AddStatement [convertAddNoGiving] [convertAddGiving]end rule

N.B. Test at every stage

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Exercises

- Add to the “commands language”:1) If “cond” then “statements” endIf2) Case cond1->statement1 … condN->statementN endCase- Transform a list of contiguos “if-statements” in a case-

statement

If x>1 then x:=x+1 endIfIf x=1 then x=0 endIfIf x<1 then x:=x-1 endIf

case x>1 -> x:=x+1; x=1 -> x=0; x<1 -> x:=x-1;end case

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Homework

- Reading “The Guided Tour” (website www.txl.ca).- Transform a case-statement in a list of contiguos “if-statements”.

case x>1 -> x:=x+1; x=1 -> x=0; x<1 -> x:=x-1;end case

If x>1 then x:=x+1 endIfIf x=1 then x=0 endIfIf x<1 then x:=x-1 endIf