The Standard ML Basis Library

The Option structure

Synopsis

signature OPTION
structure Option :> OPTION

The Option structure defines the option type, used for handling partial functions and optional values, and provides a collection of common combinators.

The type, the Option exception, and the functions getOpt, valOf, and isSome are available in the top-level environment.

Interface

datatype 'a option = NONE | SOME of 'a
exception Option
val getOpt : 'a option * 'a -> 'a
val isSome : 'a option -> bool
val valOf : 'a option -> 'a
val filter : ('a -> bool) -> 'a -> 'a option
val join : 'a option option -> 'a option
val app : ('a -> unit) -> 'a option -> unit
val map : ('a -> 'b) -> 'a option -> 'b option
val mapPartial : ('a -> 'b option)
                   -> 'a option -> 'b option
val compose : ('a -> 'b) * ('c -> 'a option)
                -> 'c -> 'b option
val composePartial : ('a -> 'b option) * ('c -> 'a option)
                       -> 'c -> 'b option

Description

datatype 'a option = NONE | SOME of 'a
The type option provides a distinction between some value and no value, and is often used for representing the result of partially defined functions. It can be viewed as a typed version of the C convention of returning a NULL pointer to indicate no value.

getOpt (opt, a)
returns v if opt is SOME(v); otherwise it returns a.

isSome opt
returns true if opt is SOME(v); otherwise it returns false.

valOf opt
returns v if opt is SOME(v); otherwise it raises the Option exception.

filter f a
returns SOME(a) if f(a) is true and NONE otherwise.

val join : 'a option option -> 'a option
The join function maps NONE to NONE and SOME(v) to v.

app f opt
applies the function f to the value v if opt is SOME(v), and otherwise does nothing.

map f opt
maps NONE to NONE and SOME(v) to SOME(f v).

mapPartial f opt
maps NONE to NONE and SOME(v) to f(v). The expression mapPartial f is equivalent to join o (map f).

compose (f, g) a
returns NONE if g(a) is NONE; otherwise, if g(a) is SOME(v), it returns SOME(f v). Thus, the compose function composes f with the partial function g to produce another partial function. The expression compose (f, g) is equivalent to (map f) o g.

composePartial (f, g) a
returns NONE if g(a) is NONE; otherwise, if g(a) is SOME(v), it returns f(v). Thus, the composePartial function composes the two partial functions f and g to produce another partial function. The expression composePartial (f, g) is equivalent to (mapPartial f) o g.

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Generated April 12, 2004
Last Modified January 22, 1997
Comments to John Reppy.

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