Next: , Previous: stack, Up: Top   [Contents]

## 74 std_util

```%--------------------------------------------------%
% vim: ft=mercury ts=4 sw=4 et
%--------------------------------------------------%
% Copyright (C) 1994-2006, 2008 The University of Melbourne.
% Copyright (C) 2016, 2018 The Mercury Team.
%--------------------------------------------------%
%
% File: std_util.m.
% Main author: fjh.
% Stability: high.
%
% This file contains higher-order programming constructs and other
% useful standard utilities.
%
%--------------------------------------------------%
%--------------------------------------------------%

:- module std_util.
:- interface.

:- import_module maybe.

%--------------------------------------------------%
%
% General purpose higher-order programming constructs
%

% compose(F, G, X) = F(G(X))
%
% Function composition.
%
:- func compose(func(T2) = T3, func(T1) = T2, T1) = T3.

% converse(F, X, Y) = F(Y, X).
%
:- func converse(func(T1, T2) = T3, T2, T1) = T3.

% pow(F, N, X) = F^N(X)
%
% Function exponentiation.
%
:- func pow(func(T) = T, int, T) = T.

% The identity function.
%
:- func id(T) = T.

%--------------------------------------------------%

% isnt(Pred, X) <=> not Pred(X)
%
% This is useful in higher order programming, e.g.
%   Odds  = list.filter(odd, Xs)
%   Evens = list.filter(isnt(odd), Xs)
%
:- pred isnt(pred(T)::in(pred(in) is semidet), T::in) is semidet.

% negate(Pred) <=> not Pred
%
% This is useful in higher order programming, e.g.
%   expect(negate(Pred), ...)
%
:- pred negate((pred)::in((pred) is semidet)) is semidet.

%--------------------------------------------------%

% maybe_pred(Pred, X, Y) takes a closure Pred which transforms an
% input semideterministically. If calling the closure with the input
% X succeeds, Y is bound to `yes(Z)' where Z is the output of the
% call, or to `no' if the call fails.
%