The KeywordFn functor
Author: John Reppy
Keywords: Standard ML, SML, SML/NJ Library

The SML of NJ Library
The `KeywordFn` functor

The `KeywordFn` functor provides a simple way to support a table of keyword (or reserved) identifiers in a scanner.

## Synopsis

    functor KeywordFn ()

## Functor Argument Interface

    type token
    type pos
    val ident : (Atom.atom * pos * pos) -> token
    val keywords : (string * ((pos * pos) -> token)) list

## Functor Argument Description

`type token`

  The type of tokens in the scanner.

`type pos`

  The type of source file positions used by the scanner (e.g., character positions in the source file).

`val ident : (Atom.atom * pos * pos) -> token`

  `ident (id, pos, pos)` is used to create an identifier token (i.e., non-keyword) for the given string, and start and end file positions.

`val keywords : (string * pos * pos) -> token list`

  A list of string-function pairs, where the strings are the keywords and the functions are used to create the corresponding scanner tokens from start and end file positions.

## Interface

        type token
        type pos
        val keyword : (string * pos * pos) -> token

## Interface Description

`type token`

  The type of tokens in the scanner.

`type pos`

  The type of source file positions used by the scanner.

`val keyword : (string * pos * pos) → token`

  `keyword (id, p1, p2)` returns the token for the identifier `id`, which is either one of the keyword tokens from the `keywords` list or otherwise is an identifier token created using the `ident` function.

## Discussion

This functor was designed for the ml-yacc scanner interface, where tokens contain their file position. It is not clear that it adds much utility over just using the `atom` type, but is maintained for backward compatibility.

## See Also

`Atom`, `AtomTable`, The Util Library

SML/NJ Version 110.99.9 (November 4, 2025)
Last updated 2025-11-04 15:48:25 UTC