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As part of the experimental setup of Miking, we currently support a way to use external libraries without interfering with the development of Miking that does not need these external dependencies.

Externals allows you to interact with code in the compilation target language from miking. Currently, externals are only available in compiled code and are in an early stage of development. The example below only covers the case where OCaml is the target language.

You can find an example of externals definitions in stdlib/ext/ and stdlib/ext/

For the sake of this example, lets say we want to define the exponential function and that miking targeting OCaml should use Float.exp from OCaml's standard library for its implementation.

We first define the external in a file under stdlib/ext, let's say stdlib/ext/, as

external externalExp : Float -> Float

which makes an external value externalExp of type Float -> Float available at the top-level. The corresponding MCore syntax is:

external ident : Type in expr

If the external has side-effects it should be annotated with a ! after the identifier, e.g.

external print ! : String -> ()

Each external identifier can only be defined once and externals cannot be partially applied.

As a temporary solution, the next step is to supply a list of implementation for our external in the language we target for compilation (in this case OCaml). We do this by creating a file stdlib/ext/ and in it we define a map from external identifiers to a list of implementations as follows:

include ""
include "ocaml/"

let mathExtMap =
use OCamlTypeAst in
mapFromSeq cmpString
("externalExp", [
expr = "Float.exp",
ty = tyarrow_ tyfloat_ tyfloat_ ,
libraries = [],
cLibraries = []

This map associates the externalExp external to a list of expressions in the target language, which here only has one element, namely the function Float.exp from OCaml's standard library. The field ty encode the OCaml type of this value (see stdlib/ocaml/, which is needed to convert values between miking and OCaml. In the case where you have multiple implementations, the compiler will try to pick the implementation which gives the least amount of overhead when converting to and from OCaml values. The libraries field list OCaml libraries that are needed to call this function, and cLibraries lists c libraries that are needed during linking. In this case none are needed since it is part of the standard library. If let's say we wanted to use Float.exp from a library foo, then we should instead have the field libraries = ["foo"]. Finally, we need to add mathExtMap to globalExternalImplsMap in stdlib/ocaml/

Conversion between values

Conversion between Miking values and OCaml values is defined in stdlib/ocaml/ Since externals are in an early stage of development these conversions are not complete and subject to change.

The following Basic types are converted without any computational overhead:

Miking typeOCaml type

The overhead of converting sequences to OCaml data-structures is proportional to the length of the sequence times the conversion cost for the type of the elements. Strings in Miking is a special case of sequences.

Miking typeOCaml typeComment
[A]'a list
[A]'a arrayA copy is made, mutating the OCaml array does not mutate the sequence.
[Char] or StringstringThe Miking string is converted to, and the OCaml string is assumed to be, encoded in UTF-8.

Tensors are passed by reference to OCaml, i.e. mutating the corresponding data-structure in OCaml will mutate the tensor. As a temporary solution the conversion depends on the underlying implementation of the tensor, which is controlled by the tensor create functions tensorCreateDense, tensorCreateCArrayInt, and tensorCreateCArrayFloat. The main purpose of the latter two is to allow data-sharing with C libraries via OCaml bindings.

Miking typeOCaml typeComment
Tensor[Int]Bigarray.((int, int_elt, c_layout) genarray.t)Must use tensorCreateCArrayInt, otherwise a runtime error occurs.
Tensor[Float]Bigarray.((float, float64_elt, c_layout) genarray.t)Must use tensorCreateCArrayFloat, otherwise a runtime error occurs.
Tensor[Int] of rank 1Bigarray.((int, int_elt, c_layout) Array1.t)Must use tensorCreateCArrayInt, otherwise a runtime error occurs.
Tensor[Float] of rank 1Bigarray.((float, float64_elt, c_layout) Array1.t)Must use tensorCreateCArrayFloat, otherwise a runtime error occurs.

Tuples are converted without overhead if the conversion of their elements are without overhead. The same is true for arrow types. The fields in Miking records are un-ordered while they are ordered in OCaml so there is some overhead involved when converting records as each field of the Miking records needs to be projected to form an new OCaml records, and vice versa. The fields of the Miking record are associated with the fields of the OCaml record by an explicit mapping defined in the * file.

If the Miking type is abstract, i.e. we define it as

type MyType

then no conversion is performed to and from this type.

Please consult stdlib/ext/ and stdlib/ext/, for more examples.


A more involved example on the use of externals is an interface to the Sundials numerical DAE solver. You find the implementation in stdlib/sundials/ and stdlib/sundials/ Note that these externals depends on the library sundialsml.

Installing this library involves first installing the Sundials C library. On ubuntu 20.04 you can do this by:

sudo apt-get install libsundials-dev

On macOS, using Homebrew, you instead do:

brew install sundials

Then install the ocaml bindings SundialsML via opam

opam install sundialsml

Currently, this library cannot be installed on the multi-core switch but you instead have to use another opam switch, e.g.

opam switch 4.12.0

After this you have to rebuild the compiler, e.g.

make clean

To run the sundials-specific test suite, use the command:

make test-sundials

To install for the current user, run make install as usual.


Another example use of externals is an interface to the constrained Non-Linear Program solver Ipopt. This interface is defined in stdlib/ipopt/ and stdlib/ipopt/ This library depends on both the OCaml library ipoptml and the ipopt c library.

To use this library you need to do the following:

Install the ipopt c library, which you can do on ubuntu 20.04 with

sudo apt-get install coinor-libipopt-dev

Install dependencies for ipoptml,

opam install ctypes ctypes-foreign

Clone the ipoptml repo and in its root run

dune build
dune install

You can then test the solver in Miking with

make test-ipopt

External Dependent Utests Pruning

As part of the parsing (see prune_external_utests in for details) utests that depend on externals are marked and removed. This done in the following steps:

  1. Dead code removal is performed to remove dead code, including any dead code inside utests that might reference externals. This is done to reduce the number of false positivities.
  2. Utests that references externals are marked and removed.
  3. Dead code removal is run again to remove any dead code that result from the removal of utests in step 2.

Additionally, if boot or mi is run without the --test flag, all utests are removed prior to dead code removal as all utests can then be considered dead code. This both allows the dead code removal to remove more dead code and simplifies the pruning of utests implementation. If any utest pruning is performed, a warning summarizing pruned utests is printed to stdout.

The pruning of utests can be disabled with the --disable-prune-utests flag and debugged with --debug-prune-utests (only in boot).

Moreover, pruning of utests can be disabled for a selection of externals, which allows for a more granular approach to the testing of external dependent code (see below).

Selective pruning

During compilation, the available OCaml packages on the current system is queried and externals depending on these packages are excluded from utests pruning. In other words, utests that depend on externals that can be compiled on the system are kept, while all others are removed. The listing of OCaml packages is based around dune installed-libraries (see externalListOcamlPackages and externalGetSupportedExternalImpls in so this functionality should not require any additional dependencies. For boot and the interpreter, all external dependent utests are removed as these currently do not support externals at all.

Test organization

make test-all runs all tests, disabling utest pruning for compiled files (i.e. if dependencies are not met, you get an error). The recepie make test-all-prune-utests runs all tests but prunes non-supported utests which is handy if your system only meet the dependencies of a subset of the externals. Interpreted files are always pruned of external dependent utests. Please consult the makefiles for more details.

Authoring new external libraries

To maintain the flexibility of the test organization, future external libraries must ensure that externals, or external dependent identifiers are only fully applied inside utests. To verify that this is the case you can just run boot eval --test <> and verify that you get a non-zero exit code on the library that you are currently working on.