Stainless Library

Stainless defines its own library with some core data types and operations on them, which work with the fragment supported by Stainless, available in frontends/library/stainless, which we encourage the reader to consult as it is always up to date.

One of the reasons for a separate library is to ensure that these operations can be correctly mapped to mathematical functions and relations inside of SMT solvers, largely defined by the SMT-LIB standard (see Thus for some data types, such as BigInt, Stainless provides a dedicated mapping to support reasoning. (If you are a fan of growing the language only through libraries, keep in mind that growing operations together with the ability to reason about them is what the development of mathematical theories is all about, and is a process slower than putting together libraries of unverified code–efficient automation of reasoning about a single decidable theory generally results in multiple research papers.) For other operations (e.g., List[T]), the library is much like Stainless user-defined code, but specifies some useful preconditions and postconditions of the operations, thus providing reasoning abilities using mechanisms entirely available to the user.


The ScalaDoc for the library is available online.

For the most up-to-date version of the source code of library, please consult the library/ directory in your Stainless distribution.

To use Stainless’ libraries, you need to use the appropriate import directive at the top of Stainless’ compilation units. Here is a quick summary of what to import.

Package to import

What it gives access to


Stainless annotations, e.g. @induct


Map, Set, PartialFunction, holds, passes, invariant


List[T] and subclasses, Option[T] and subclasses

To learn more, we encourage you to look in the library/ subdirectory of Stainless distribution.


Stainless provides some special annotations in the package stainless.annotation, which instruct Stainless to handle some functions or objects in a specialized way.




Treat this object/function as library, don’t try to verify its specification. Can be overridden by including a function name in the --functions command line option.


Use the inductive tactic when generating verification conditions.


Treat the annotated method as an invariant of the enclosing class. Can be used instead of require within a value class body. For soundness, invariants can only refer to fields of their class, and thus cannot call methods on this.


Drop the annotated field or method during compilation. See the corresponding section for more information.


Only extract the contracts of a function, replacing its body by a choose expression.


Used to hide a function f’s body when doing verification of functions (f itself, or others) invoking f. Does not hide pre and postconditions.


Do not generate verification conditions for this function.


Specify that this function is pure, which will then be checked. If the function is also annotated with @extern, it will not be checked, but assumed pure.


Ignore this definition when extracting Stainless trees. This annotation is useful to define functions that are not in Stainless’s language but will be hard-coded into specialized trees, or to include code written in full Scala which is not verifiable by Stainless. Can also be used on class fields whose type cannot be understood by Stainless, eg. because it comes from an external library, the JDK, or some other code which does not understand. See the corresponding documentation page.


Inline this function. Stainless will refuse to inline (mutually) recursive functions.


Inline this function but only once, which is allowed even on (mutually) recursive functions. Note: A recursive function will not be inlined within itself.


Partially evaluate calls to this function. Note: stainless.lang.partialEval can also be used to partially evaluate an expression.

Stainless also has some special keywords defined in stainless.lang that can be used around function calls. Here is an example for unfold.




Call-site inlining


Inject an equality assumption between a function call and its unfolded version. Can be useful to locally override an @opaque annotation.


As there is no special support for Lists in SMT solvers, Stainless Lists are encoded as an ordinary algebraic data type:

sealed abstract class List[T]
case class Cons[T](h: T, t: List[T]) extends List[T]
case class Nil[T]() extends List[T]

List API

Stainless Lists support a rich and strongly specified API.

Method signature for List[T]

Short description

size: BigInt

Number of elements in this List.

content: Set[T]

The set of elements in this List.

contains(v: T): Boolean

Returns true if this List contains v.

++(that: List[T]): List[T]

Append this List with that.

head: T

Returns the head of this List. Can only be called on a nonempty List.

tail: List[T]

Returns the tail of this List. Can only be called on a nonempty List.

apply(index: BigInt): T

Return the element in index index in this List (0-indexed).

::(t:T): List[T]

Prepend an element to this List.

:+(t:T): List[T]

Append an element to this List.

reverse: List[T]

The reverse of this List.

take(i: BigInt): List[T]

Take the first i elements of this List, or the whole List if it has less than i elements.

drop(i: BigInt): List[T]

This List without the first i elements, or the Nil() if this List has less than i elements.

slice(from: BigInt, to: BigInt): List[T]

Take a sublist of this List, from index from to index to.

replace(from: T, to: T): List[T]

Replace all occurrences of from in this List with to.

chunks(s: BigInt): List[List[T]]

zip[B](that: List[B]): List[(T, B)]

Zip this list with that. In case the Lists do not have equal size, take a prefix of the longer.

-(e: T): List[T]

Remove all occurrences of e from this List.

--(that: List[T]): List[T]

Remove all occurrences of any element in that from this List.

&(that: List[T]): List[T]

A list of all elements that occur both in that and this List.

pad(s: BigInt, e: T): List[T]

Add s instances of e at the end of this List.

find(e: T): Option[BigInt]

Look for the element e in this List, and optionally return its index if it is found.

init: List[T]

Return this List except for the last element. Can only be called on nonempty Lists.

last: T

Return the last element of this List. Can only be called on nonempty Lists.

lastOption: Option[T]

Optionally return the last element of this List.

headOption: Option[T]

Optionally return the first element of this List.

unique: List[T]

Return this List without duplicates.

splitAt(e: T): List[List[T]]

Split this List to chunks separated by an occurrence of e.

split(seps: List[T]): List[List[T]]

Split this List in chunks separated by an occurrence of any element in seps.

count(e: T): BigInt

Count the occurrences of e in this List.

evenSplit: (List[T], List[T])

Split this List in two halves.

insertAt(pos: BigInt, l: List[T]): List[T]

Insert an element after index pos in this List.

replaceAt(pos: BigInt, l: List[T]): List[T]

Replace the l.size elements after index pos, or all elements after index pos if there are not enough elements, with the elements in l.

rotate(s: BigInt): List[T]

Rotate this list by s positions.

isEmpty: Boolean

Returns whether this List is empty.

map[R](f: T => R): List[R]

Builds a new List by applying a predicate f to all elements of this list.

foldLeft[R](z: R)(f: (R,T) => R): R

Applies the binary operator f to a start value z and all elements of this List, going left to right.

foldRight[R](f: (T,R) => R)(z: R): R

Applies a binary operator f to all elements of this list and a start value z, going right to left.

scanLeft[R](z: R)(f: (R,T) => R): List[R]

Produces a List containing cumulative results of applying the operator f going left to right.

scanRight[R](f: (T,R) => R)(z: R): List[R]

Produces a List containing cumulative results of applying the operator f going right to left.

flatMap[R](f: T => List[R]): List[R]

Builds a new List by applying a function f to all elements of this list and using the elements of the resulting Lists.

filter(p: T => Boolean): List[T]

Selects all elements of this List which satisfy the predicate p

forall(p: T => Boolean): Boolean

Tests whether predicate p holds for all elements of this List.

exists(p: T => Boolean): Boolean

Tests whether predicate p holds for some of the elements of this List.

find(p: T => Boolean): Option[T]

Finds the first element of this List satisfying predicate p, if any.

takeWhile(p: T => Boolean): List[T]

Takes longest prefix of elements that satisfy predicate p

Additional operations on Lists

The object ListOps offers this additional operations:

Function signature

Short description

flatten[T](ls: List[List[T]]): List[T]

Converts the List of Lists ls into a List formed by the elements of these Lists.

isSorted(ls: List[BigInt]): Boolean

Returns whether this list of mathematical integers is sorted in ascending order.

sorted(ls: List[BigInt]): List[BigInt]

Sorts this list of mathematical integers is sorted in ascending order.

insSort(ls: List[BigInt], v: BigInt): List[BigInt]

Sorts this list of mathematical integers is sorted in ascending order using insertion sort.

Theorems on Lists

The following theorems on Lists have been proven by Stainless and are included in the object ListSpecs:

Theorem signature

Proven Claim

snocIndex[T](l: List[T], t: T, i: BigInt)

(l :+ t).apply(i) == (if (i < l.size) l(i) else t)

reverseIndex[T](l: List[T], i: BigInt)

l.reverse.apply(i) == l.apply(l.size - 1 - i)

appendIndex[T](l1: List[T], l2: List[T], i: BigInt)

(l1 ++ l2).apply(i) == (if (i < l1.size) l1(i) else l2(i - l1.size))

appendAssoc[T](l1: List[T], l2: List[T], l3: List[T])

((l1 ++ l2) ++ l3) == (l1 ++ (l2 ++ l3))

snocIsAppend[T](l: List[T], t: T)

(l :+ t) == l ++ Cons[T](t, Nil())

snocAfterAppend[T](l1: List[T], l2: List[T], t: T)

(l1 ++ l2) :+ t == (l1 ++ (l2 :+ t))

snocReverse[T](l: List[T], t: T)

(l :+ t).reverse == Cons(t, l.reverse)

reverseReverse[T](l: List[T])

l.reverse.reverse == l

scanVsFoldRight[A,B](l: List[A], z: B, f: (A,B) => B)

l.scanRight(f)(z).head == l.foldRight(f)(z)

Set[T], Map[T]

Stainless uses its own Sets and Maps, which are defined in the stainless.lang package. However, these classes are not implemented within Stainless. Instead, they are parsed into specialized trees. Methods of these classes are mapped to specialized trees within SMT solvers. For code generation, we rely on Java Sets and Maps.

The API of these classes is a subset of the Scala API and can be found in the Pure Scala section.

Additionally, the following functions for Sets are provided in the stainless.collection package:

Function signature

Short description

setToList[A](set: Set[A]): List[A]

Transforms the Set set into a List.

setForall[A](set: Set[A], p: A => Boolean): Boolean

Tests whether predicate p holds for all elements of Set set.

setExists[A](set: Set[A], p: A => Boolean): Boolean

Tests whether predicate p holds for all elements of Set set.

PartialFunction[A, B]

To define anonymous functions with preconditions, Stainless has a PartialFunction[A, B] type with the corresponding annotation A ~> B. To construct a partial function, you must use PartialFunction.apply as in the unOpt function below. The precondition written in the require becomes the pre field of the partial function (as in the call to f.pre in map1).

def map1[A, B](l: List[A], f: A ~> B): List[B] = {
  l match {
    case Cons(x, xs) => Cons[B](f(x), map1(xs, f))
    case Nil() => Nil[B]()

def unOpt[T](l: List[Option[T]]): List[T] = {
  map1(l, PartialFunction {(x:Option[T]) => require(x.nonEmpty); x.get})

Partial functions can also be written using pattern matching:

def unOptCase[T](l: List[Option[T]]): List[T] = {
  require(l.forall { case Some(_) => true; case _ => false })
  map1(l, PartialFunction[Option[T], T] { case Some(v) => v })