# ExtractionExtracting ML from Coq

# Basic Extraction

Extraction Language Ocaml.

Now we load up the Coq environment with some definitions, either
directly or by importing them from other modules.

Finally, we tell Coq the name of a definition to extract and the
name of a file to put the extracted code into.

Extraction "imp1.ml" ceval_step.

When Coq processes this command, it generates a file imp1.ml
containing an extracted version of ceval_step, together with
everything that it recursively depends on. Compile the present
.v file and have a look at imp1.ml now.

# Controlling Extraction of Specific Types

- how the Coq type itself should be represented in OCaml, and
- how each constructor should be translated.

Also, for non-enumeration types (where the constructors take
arguments), we give an OCaml expression that can be used as a
"recursor" over elements of the type. (Think Church numerals.)

Extract Inductive nat ⇒ "int"

[ "0" "(fun x → x + 1)" ]

"(fun zero succ n →

if n=0 then zero () else succ (n-1))".

[ "0" "(fun x → x + 1)" ]

"(fun zero succ n →

if n=0 then zero () else succ (n-1))".

We can also extract defined constants to specific OCaml terms or
operators.

Extract Constant plus ⇒ "( + )".

Extract Constant mult ⇒ "( * )".

Extract Constant beq_nat ⇒ "( = )".

Extract Constant mult ⇒ "( * )".

Extract Constant beq_nat ⇒ "( = )".

Important: It is entirely

*your responsibility*to make sure that the translations you're proving make sense. For example, it might be tempting to include this one
Extract Constant minus ⇒ "( - )".

but doing so could lead to serious confusion! (Why?)
Extraction "imp2.ml" ceval_step.

Have a look at the file imp2.ml. Notice how the fundamental
definitions have changed from imp1.ml.

# A Complete Example

We also need one more variant of booleans.

The extraction is the same as always.

Now let's run our generated Imp evaluator. First, have a look at
impdriver.ml. (This was written by hand, not extracted.)
Next, compile the driver together with the extracted code and
execute it, as follows.

ocamlc -w -20 -w -26 -o impdriver imp.mli imp.ml impdriver.ml ./impdriver(The -w flags to ocamlc are just there to suppress a few spurious warnings.)