if then else in ocamlyacc

Question

Can anyone brief how can I implement if then else in Ocamlyacc. I've defined tokens from lexical analyzer(ocamllex) namely IF, THEN, ELSE. For the if statement, I have defined tokens: GREATERTHAN, LESSERTHAN, EQUALTO for integers. I've searched many tutorials but to no avail!

UPDATE:

I want to interpret the result and return the value of the expression dictated by the if-else statement.

score 3 · Accepted Answer · answered Jul 07 '13 at 22:53

3

You have to define rules :

ifthenelse :    
 |   IF condition THEN statement ELSE statement   { IfThenElse($1,$2,$3) }


condition :    
| INT EQUALTO INT  { Cond(EqualTo,$1,$3) }   
| INT LESSERTHAN INT {  Cond(LesserThan,$1,$3) }   
| INT GREATERTHAN INT {  Cond(GeaterThan,$1,$3) }

Don't forget to define regular expression for int, in your lex fil

answered Jul 07 '13 at 22:53

Ontologiae

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Could you also define the functions IfThenElse , Cond for me? Thanks in advance! – user2352241 Jul 08 '13 at 12:16
It's just a type you define : Cond of op * int * int etc. – Ontologiae Jul 08 '13 at 15:07
Can I use Ocaml to evaluate this - like an interpreter? – user2352241 Jul 08 '13 at 16:20
You can very easily write an interpreter for your abstract syntax, but it doesn't come free. – Jeffrey Scofield Jul 08 '13 at 17:46
@JeffreyScofield: I suppose that will require me to write a function that takes input from ocamlyacc and executes the required action. – user2352241 Jul 09 '13 at 12:47
This sounds right. The function would consume the output of the parser and evaluate it. It sounds complicated but once you start coding it's a breeze (in my experience). – Jeffrey Scofield Jul 09 '13 at 18:39
You can see a simple example here (there's a make top for top level playing) https://github.com/ontologiae/DRS-Analysis-Tool – Ontologiae Jul 12 '13 at 13:33

score 0 · Answer 2 · answered Jul 10 '13 at 06:35

Perhaps you've seen it, but the OCaml manual gives a complete ocamllex/ocamlyacc example that calculates the values of expressions: Desk Calculator Example.

The example shows that you can calculate your result in the ocamlyacc actions if you want to. For a simple example, it's not at all hard to follow. In a more realistic case, you would probably want to construct an abstract syntax tree for later processing (such as evaluation). The code has a similar flavor except that the cases are given by the different constructors of your AST type rather than by the different grammar rules.

if then else in ocamlyacc

2 Answers2