Istari employs two types for terms: Term.term
is the internal
representation of terms, and ETerm.eterm
is the external
representation. We usually refer to both of them as just terms,
except when the context requires us to distinguish them.
The main difference between internal and external terms is that internal terms represent variables using de Bruijn indices, while external terms use explicit names. (Istari’s de Bruijn indices count from zero.) External terms are converted to internal ones by supplying a data structure (called a directory) that maps names onto indices. (A few other things are done while converting external terms to internal ones as well.)
The default grammar for terms is as follows. Capitalized words are
nonterminals; lower case words are keywords. (Exception: U
and
Kind
are keywords.)
Forms are listed in increasing order of precedence. Grouped forms
have the same precedence. A subterm at the same level of precedence
is given in brackets. For example, [Term] + Term
indicates that
addition is left associative, while Term :: [Term]
indicates that
cons is right associative.
Term ::=
fn Bindingsn . [Term] (lambda)
forall Bindings . [Term] (dependent product)
exists Bindings . [Term] (strong dependent sum)
iforall Bindings . [Term] (impredicative universal)
iexists Bindings . [Term] (impredicative existential)
intersect Bindings . [Term] (intersection type)
union Bindings . [Term] (intersection type)
foralltp OIdents . [Term] (impredicative polymorphism)
rec Ident . [Term] (recursive type)
mu Ident . [Term] (inductive type)
wtype ( Ident : [Term] ) . [Term] (W type)
iset ( Ident : [Term] ) . [Term] (intensional set type)
quotient ( Ident Ident : [Term] ) . [Term] (quotient type)
Term > [Term] (ordinary arrow)
Term t> [Term] (tarrow kind)
Term k> [Term] (karrow kind)
Term g> [Term] (guard)
let Ident = Term in [Term] (let)
let next Ident = Term in [Term] (scoped future elim)
case Term of  inl OIdent . [Term]  inr OIdent . [Term] (sum elim)
Term : [Term] (typing)
Term : type (type formation)
fnind ... inductive function ... (inductive function)
if [Term] then [Term] else [Term] (ifthenelse)
Term <> Term (ifandonlyif)
Term % [Term] (sum type)
Term & [Term] (product type)
Term = Term : Term (equality)
Term = Term : type (type equality)
Term <: Term (subtyping)
Term <:> Term (extensional type equivalence)
Term != Term : Term (inequality)
Term <= Term (natural number inequality)
Term < Term (natural number inequality)
Term <l= Term (level inequality)
Term <z= Term (integer inequality)
Term <z Term (integer inequality)
Term :: [Term] (cons)
[Term] + Term (natural number plus)
[Term]  Term (natural number minus)
[Term] +z Term (integer plus)
[Term] z Term (integer minus)
[Term] * Term (natural number times)
[Term] *z Term (integer times)
[Term] Term (application)
[Term] #1 (first projection)
[Term] #2 (second projection)
[Term] #prev (unscoped future elim)
[Term] ## Number (#2 ... #2 #1)
pi1 Term (first projection)
pi2 Term (second projection)
prev Term (unscoped future elim)
next Term (future intro)
inl Term (sum intro)
inr Term (sum intro)
~z Term (integer negation)
U Level (universe)
Kind Level (kind)
[Term] :> Term (type annotation)
[Term] ap Term (visibilized application)
[Term] _# Number (application to multiple evars)
[Term] __# Number (application to multiple holes)
( Term )
() (unit)
( Term , ... , Term ) (tuple, length at least 2)
{ Ident : Term  Term } (set type)
{ Term } (squashed type)
Longident (identifier)
Number (natural number literal)
_ (evar)
__ (hole)
\ ... antiquoted internal term ... \
` Longident
l` LTerm (literal term)
level` Level (level expression)
z` Number (integer literal)
OIdent ::=
Ident
_ (anonymous argument)
OIdents ::=
OIdent ... OIdent (length can be zero)
Binding ::=
OIdent (unannotated binding)
(OIdents : Term) (binding with type supplied)
Bindings ::=
Binding ... Binding (length at least 0)
Bindingsn ::=
Binding ... Binding (length at least 1)
Level ::=
Number + [Level] (level plus)
( Level )
Number (level literal)
[ Level ... Level ] (level max, length at least 1)
Ident (identifier)
\ ... antiquoted internal term ... \
Notes:
Note that typing (M : A
) has lower precedence than equality
(M = N : A
). Thus, M : A > B
asserts that M
has the type
A > B
, while M = N : A > B
asserts that the equality
M = N : A
implies B
.
Similarly, type formation (A : type
) has lower precedence than type
equality (A = B : type
). Thus A : type > C
is an error (since
type > C
is illegal), while A = B : type > C
asserts that the
equality A = B : type
implies C
.
The pair form is (Term , Term)
, not (Term, Term)
or
(Term,Term)
. (Observe the whitespace on both sides of the comma.)
For example, because of how IML tokenizes
input, the x,
in (x, y)
forms a single
token, rather than two, which will result in a syntax error.
A tick (`
) before an identifier suppresses the insertion of
implicit arguments. (See below.) It also allows the identifier to
be any constant name, even if the name collides with a keyword.
An antiquoted internal term should have the
type Term.term
.
A hole (__
) is a placeholder used by some tactics (e.g., so
).
It is written as two consecutive underscores.
One can put multiple anonymous arguments into a Bindings
or
Bindingsn
by writing _# Number
. The multiplicity can be zero.
(The requirement that a Bindingsn
be nonempty can be defeated
using a zero multiplicity.)
One can suppress the insertion of implicit arguments throughout an
entire term using the syntax explicit` Term
. The inner term
should be parenthesized or otherwise syntactically atomic (that is,
it should appear in the last grouping of Term
syntax).
One can bind additional de Bruijn positions using the syntax
additional` OIdents . Term
. For example, additional` x y . y
resolves to index 0; and if z
resolves to index 3, then
additional` x y . z
resolves to index 5.
One can exclude a variable from appearing within a term using the
syntax exclude` Ident in Term
. This is occasionally useful for
pruning evar dependencies, to assist unification.
The syntax for inductive functions appears here.
The LTerm
(“literal term”) grammar represents internal terms
directly. It uses de Bruijn indices rather than names and offers very
few conveniences. It also provides a notation for explicit
substitution.
LTerm ::=
fn . [LTerm] (lambda)
LTerm = LTerm : LTerm (equality)
LTerm = LTerm : type (type equality)
[LTerm] LTerm (application)
[LTerm] #1 (first projection)
[LTerm] #2 (second projection)
[LTerm] #prev (unscoped future elim)
next LTerm (future intro)
[LTerm] [ Sub ] (substitution)
( LTerm )
() (unit)
( LTerm, LTerm ) (pair)
U (universe w/o argument)
Number (de Bruijn index)
\ ... antiquoted internal term ... \
` Longident
z` Number (integer literal)
Sub ::=
[Sub] o Sub (composition)
LTerm . [Sub] (substitution cons)
Number . [Sub] (special case of cons)
^ Number (shift)
id (identity, same as ^ 0)
( Sub )
Notes:
The innermost de Bruijn index is 0.
Literal terms do not insert implicit arguments. Nevertheless, the
tick form (`
) is still useful for using constants whose names
collide with a keyword.
In the internal representation for explicit substitutions, there is a special case of substitution cons for when the term being consed is an index.
Most of the nonterminals above can be parsed by various pervasive
functions: parseIdent
, parseIdents
, parseLongident
,
parseOIdent
, parseOIdents
, parseBinding
, parseBindings
,
parseLTerm
, parseSub
.
These functions are all actually the identity function, but IML is instructed to parse their argument using the indicated nonterminal.
Some constants take implicit and/or invisible arguments:
Implicit arguments are not seen by the user, but they are present in the real term “under the hood.” The process of converting external terms to internal terms inserts evars for implicit arguments, which are then usually resolved by unification.
If the user wants to suppress the insertion of implicit arguments
(for example, because one wants to give them explicitly), one can
precede the constant with a tick (`
).
Invisible arguments, on the other hand, are not present in the real
term at all, even “under the hood.” They arise when a function has
a type like intersect (x : A) . B
. Such a function has the type
B
for any choice for x
of type A
, but that choice is not
passed in to the function at all.
Sometimes it is necessary to specify a function’s invisible argument
because typechecking is unable to work it out itself. To do so, one
employs visibilized application by writing f ap m
to indicate that
the f
’s invisible argument should be filled in with m
.
Visibilized application is only a hint for the type checker; such
arguments are still not passed into the function. Thus, f ap m
unfolds to just f
.
For example, List.map
has type:
intersect i . forall (a b : U i) . (a > b) > list a > list b
and has two implicit arguments. Thus, its first argument (i
) is
invisible, and its second and third arguments (a
and b
) are
implicit. Normally one calls List.map
with just the explicit
arguments; so List.map F L
means:
`List.map _ _ F L
If, instead, one wanted to supply all the arguments, one would write something like:
`List.map ap I A B F L
The ap I
fills in the invisible argument, while the starting tick
suppresses the insertion of evars for implicit arguments, so A
and
B
can be supplied.
Note that implicit arguments are inserted first, so
List.map ap I F L
would mean:
`List.map _ _ ap I F L`
which is probably not desired. But rarely would one want to give only the invisible arguments explicitly anyway.
Constants have one of five levels of opacity: soft, firm, softstrict, hard, or opaque:
OPAQUE

HARD
/ \
FIRM SOFT_STRICT
\ /
SOFT
Hard: the default opacity for a constant with a definition. The
constant can be unfolded, and a few tactics (notably intro
and
destruct
) will unfold it automatically. It is never unfolded
automatically by unification. It is not unfolded in typechecking,
except that it is unfolded when necessary to infer the type of a
spine. For instance, when inferring the type of f x y
, if the
type of f x
is a hard constant, that constant will be unfolded so
inference can continue with the next argument.
Opaque: the constant cannot be unfolded. Any constant without a definition is opaque, but not vice versa; opaque constants can have definitions.
Soft: the constant is automatically unfolded by unification, typechecking, and a variety of tactics. Soft constants are best viewed as abbreviations. Giving a type to a soft constant usually serves no purpose (other than documentation) because the typechecker automatically unfolds it before determining its type.
Firm: like a soft constant, except the constant is not unfolded by
the typechecker in the subject position. Thus, a firm constant can
be given a type for the typechecker to use. This is typically used
for constants whose purpose is to give
assistance to the typechecker
(ann
, ap
, fnann
, manualf
).
Softstrict: like a soft constant, except that when unification is presented with a constraint equating two terms whose head are the same softstrict constant, unification will unify the corresponding terms along the spine, rather than unfolding the constant.
Note that Istari does not check that this is correct. Marking a constant as softstrict that is not actually strict will not compromise soundness, but it will cause unification to fail sometimes when it need not fail.
A constant’s opacity can be altered using the function setOpacity
.
Since opacities can be altered, an opaque constant is not entirely
abstract. To make a constant abstract, one may delete a constant’s
definition, thereby rendering it permanently opaque, using the
function abstract
.
(* abbreviated *)
signature CONSTANT =
sig
...
datatype opacity = SOFT  FIRM  SOFT_STRICT  HARD  OPAQUE
(* returns the constant's definition if the constant is not OPAQUE *)
val definition : constant > term option
val opacity : constant > opacity
val setOpacity : constant > opacity > unit
(* sets opacity to OPAQUE and deletes the definition (cannot be reversed) *)
val abstract : constant > unit
end
structure Constant : CONSTANT