IML is an ML dialect close to Standard ML. Most SML code should require very little modification to serve as IML code.

The IML preprocessor can generate either SML or OCaml. Compatibility with OCaml drives many of the differences between IML and SML listed here.

User-definable parsing

The most visible difference between IML and SML is user-definable parsing. For example, to begin a lemma in Istari, one might enter:

lemma "reflexivity" / forall (n : nat) . n = n : nat /;

Here the lemma function has type string -> ETerm.eterm -> unit. The code within the slashes is passed to a user-definable parser, in this case a parser for terms. The term parser is the default, but every (short) identifier can be assigned a different parser for each of its arguments. (One can also change the default parser, but that is not recommended when using Istari.)

For example, the defineRaw function takes a “definition pattern” as its first argument and a term as its second:

defineRaw /double n/ /n + n/;

Many user-definable parsers all you to embed an arbitrary ML term into the parse. This is done using an antiquote mechanism, where the embedded term is enclosed by backslashes. (If the embedded term is inappropriate, this may result in a type error.)

For example, to write a ML function that turns any Istari term into its successor, one writes:

fn t => / succ \t\ /

Input code is tokenized before it is passed to a user-definable parser. (This means that user-definable parsers need not deal with lexical analysis or whitespace.) Tokenization is done in a simple-minded matter since it does not know anything about the grammar.

The main rule is that tokens are separated by whitespace. There are three exceptions:

Thus, one should probably not write (x, y), because the x, forms a single token. Instead write (x , y).

Proof management syntax

The second-most visible difference between IML and SML is special syntax for managing proofs. One applies a tactic to the current goal by ending the tactic code with a period, and one can enter and leave subgoals with curly braces:

Syntax Elaborates to:
[tactic]. Prover.apply [tactic];
{ Prover.enter ();
} Prover.leave ();
[number]:{ Prover.entern [number];

Anonymous multi-argument functions

IML supports syntax for anonymous multi-argument functions. A curried function taking three arguments can be written:

fns x y z => [body]

This is equivalent to:

fn x => fn y => fn z => [body]

Unlike fn, neither fns cannot accept multiple clauses.

Collapsed tuples

A left-associated iterated pair can be written using the quasi-constructor Collapse:

Collapse (1, 2, 3)

is equivalent to:

((((), 1), 2), 3)

This can be used with patterns as well.

There is also special syntax for a function taking a collapsed tuple:

fnc x y z => [body]

is equivalent to:

fn (Collapse (x, y, z)) => [body]

which in turns means:

fn ((((), x), y), z) => [body]

The fnc form is mainly used to interact with Istari’s case-analysis module Case. Unlike fn, fnc cannot accept multiple clauses.


IML exception handling is written using “try-with” rather than “handle”. Thus, instead of:

[term] handle [constructor] => [handler-term]

one writes:

try [term] with [constructor] => [handler-term]

Do bindings

An IML let binding can employ do bindings to sequence operations:

   do x = exp1

All of the let that follows the do is wrapped up as a function and passed as an additional argument to exp1, producing:

exp1 (fn x => exp2)

One use of this device is for writing monadic code. For example, andthenM is the bind operation for the tacticm monad:

   do x = andthenM tac1
   do y = andthenM tac2

This expression is elaborated to:

andthenM tac1 (fn x => andthenM tac2 (fn y => tac3))

Another use is for writing code in continuation-passing style, such as:

   do m = withterm / [term] /

This elaborates to:

withterm / [term] / (fn m => tac)

The Istari tactic library is implemented using continuation-passing style, so it makes considerable use of do.


In IML, records are permitted only as arguments to datatype constructors. Thus:

type point = { x : int, y : int }

is not permitted, but:

datatype point = Point of { x : int, y : int }

is permitted.

Since the fields of a record can always be determined from the constructor, there is no need for wildcard patterns. So instead of:

val Point { x=horiz, ... } = [term]

one just writes:

val Point { x=horiz } = [term]

Datatype patterns

In IML (like OCaml), one cannot match multiple datatype arguments using a single variable. Thus:

datatype exp = App of exp * exp | ...

fun eval (App e) = ...

is not permitted. One must write:

fun eval (App (e1, e2)) = ...

instead. However, one is permitted to match multiple datatype arguments with a single wildcard (_).

Record/tuple projection

SML’s record and tuple projection syntax (e.g., #1) is not supported. But for pairs and triples the basis provides substitutes: fst, snd, n1of3, n2of3, n3of3.

Unbounded integer literals

An unbounded integer literal (of type can be written using an I suffix, such as 0I, 12I, or ~12I.

Suppressing infix

An infix operator can be used as an ordinary function by placing it within parentheses. For example, (+) : int -> int -> int. (In SML this would be written op +.) For operators beginning or ending with an asterisk, one must add extra space(s) to prevent the parser from interpreting it as a comment (e.g., ( * )).


IML’s string escapes are:

escape meaning
\n newline
\" quotation mark
\\ backslash
\x[two hex digits] indicated ASCII character
\[nonempty whitespace]\ omitted

Other omissions

Some other features of SML are unsupported:

The basis

IML has its own standard basis (inspired by the SML basis, but different from it).