By Jacob Neumann, May 2020

bool is the SML type of booleans. The bool type supports the usual constructs of boolean logic and "conditionals" (if expressions). bool is also the type produced when evaluating (in)equality between values of (suitable) types.


There are exactly two values of type bool, true and false.

datatype bool = true | false

In addition to the constructs generally available in pattern matching (e.g. wildcards and identifier binding), booleans can be pattern-matched against using the constructors true and false.

fun firstOrSecond ((x : int,y : int), true):int = x
  | firstOrSecond ((x,y), false) = y

val 2 = firstOrSecond((3,2),false)

bool is pretty-printed by the smlnj REPL, so the actual values will display. This is demonstrated by the following smlnj REPL snippet.

- val b = true orelse false;
val b = true : bool


Some common functions which produce booleans:

(op =)  : ''a * ''a -> bool
(op <>) : ''a * ''a -> bool  (* Inequality *)

(* All of the following are overloaded and also work on values of type real *)
(op <)  : int * int -> bool
(op >)  : int * int -> bool
(op <=) : int * int -> bool
(op >=) : int * int -> bool


The principal use of booleans is for evaluating one of two possible expressions, conditional on a value of type bool:

(* Evaluates to 5 *)
val res1 = if true then 5 else 2

(* Evaluates to 7 *)
val res2 = if false then 3+3 else 7

Note that the expression between then and else (the "then branch") has the same type as the expression after the else (the "else branch"). This is necessary: the SML typechecker does not evaluate expressions, and so does not "know" that, for instance, if false then e1 else e2 will always reduce to e2. As far as the typechecker knows, if false then e1 else e2 could reduce to e1. So, in order for the typechecker to be able to assign a type to the expression if false then e1 else e2, it must be the case that e1 and e2 have the same type. More formally,

[If-Then] An expression if b then e1 else e2 is well-typed (with type t) if and only if b : bool and e1 : t and e2 : t.

It is worth noting that if b then e1 else e2 is equivalent to the following expression, written using SML's case syntax.

    case b of
      true => e1
    | false => e2

Which is also equivalent to

    (fn true => e1 | false => e2) b

NOTE: It's important to note that, in the evaluation of the expression if true then e1 else e2, the expression e2 is never evaluated (and, analogously, e1 never is evaluated in if false then e1 else e2). This is most evident when we look at the third syntax (with the lambda function): SML does not evaluate the body of a function until the function is called. So when (fn true => e1 | false => e2) gets applied to, say, true, then the evaluation steps immediately to e1, without ever evaluating e2. This point is explored more in a question below.


bool is an equality type, and may therefore be compared with =, producing another bool.

val true = (true = true)
val false = (true = false)
val false = (false <> false)

bool also comes equipped with the usual boolean operators,

val true = true andalso true   (* andalso keyword, logical and *)
val true = false orelse true   (* orelse keyword, logical or *)
val false = not true           (* not:bool -> bool, logical negation *)

An important note about andalso and orelse: the evaluation of b1 andalso b2 has a behavior known as short-circuiting:[1] when evaluating this expression, SML will first attempt to evaluate b1. If b1 raises an exception or loops behavior, then that will be the behavior of b1 andalso b2 as a whole. If b1 reduces down to the value true, then SML will then attempt to evaluate b2. However, if b1 evaluates to the value false, then SML will not evaluate b2. This is exhibited in the following code snippet.

(* loops forever on any input *)
fun loop (x:int):bool = loop x

(* Evaluates to the value false, and doesn't loop *)
val false = false andalso (loop 3)

From the Structure

The structure Bool is bound as part of the SML Basis. In addition to what's already been mentioned, it includes the utility function

	Bool.toString : bool -> string

This is useful (for instance) for print-debugging the value of a bool-valued variable.

Questions to Consider

  1. Why are the following expressions not equivalent?
    (if b then e1 else e2)

    (fn (x,y) => if b then x else y) (e1,e2)
  1. Why are the following expressions not equivalent?
    b1 andalso b2

    (fn (v1,v2) => v1 andalso v2) (b1,b2)


[1]: This is why andalso and orelse are designated as keywords above: they are not infixed functions of type bool*bool->bool. Functions cannot exhibit this kind of "shortcircuiting" (evaluating one of their arguments and then deciding whether to evaluate the other): the integer addition (op +) : int*int -> int must have both of its arguments fully evaluated before proceeding to add them. The keywords andalso and orelse must be built-in to the SML evaluator to achieve shortcircuiting.