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In lazy functional languages, a refutable pattern is one which may fail to match. An expression being matched against a refutable pattern is first evaluated to head normal form (which may fail to terminate) and then the top-level constructor of the result is compared with that of the pattern. If they are the same then any arguments are matched against the pattern's arguments otherwise the match fails.

An irrefutable pattern is one which always matches. An attempt to evaluate any variable in the pattern forces the pattern to be matched as though it were refutable which may fail to match (resulting in an error) or fail to terminate.

Patterns in Haskell are normally refutable but may be made irrefutable by prefixing them with a tilde (~). For example,

(\ (x,y) -> 1) undefined ==> undefined (\ ~(x,y) -> 1) undefined ==> 1

Patterns in Miranda are refutable, except for tuples which are irrefutable. Thus

g [x] = 2 g undefined ==> undefined

f (x,y) = 1 f undefined ==> 1

Pattern bindings in local definitions are irrefutable in both languages:

h = 1 where [x] = undefined ==> 1 Irrefutable patterns can be used to simulate unlifted products because they effectively ignore the top-level constructor of the expression being matched and consider only its components.
References in periodicals archive ?
Cela nous amene neanmoins a nous demander si le droit serait mieux servi si l'on reconnaissait I'existence d'une presomption refutable d'absence d'intention de cette nature dans des circonstances ou il y a echange rapide de courriels entre deux consommateurs agissant seuls.
In the next section, we examine data that enable us to test the following refutable hypotheses:
En el sistema LT, se formalizan las nociones meta-logicas de tautologia, contradiccion, satisfacible, refutable y contingencia.
Of course I am incapable of looking, touching, I feel responsible for this thing, and thus, refutable.
Therefore, being ready for research as a profession also means that, by nature, good news or bad news related to chiropractic knowledge is always temporary, partial, debatable and of course refutable.
After referring to Popper as describing logic and mathematics as empirically refutable insofar as they are applied to reality (something which I think that he slightly misconstrues (9), Hindess writes: "if logic, other theories, and singular descriptive statements are all refutable in principle then no test can ever provide a logically conclusive refutation of any one universal statement.
El racionalismo critico de Popper se caracteriza porque plantea un conocimiento que es cientifico porque es provisional, no definitivo, falible, no dogmatico, objetivo (linguisticamente formulable), no violento, no valido a priori (geneticamente a priori, pero valido a posteriori, es decir, tras someter el conocimiento a critica) y refutable, falsable (sometible a critica).
La hipotesis del asesinato de Bolivar parece verificable, pero no parece refutable.
But there is a far more disturbing (and less examined) claim embedded in Beck's film, which has much farther-reaching and damaging repercussions for Jews than the easily refutable international conspiracy mumbo-jumbo seconded by the intelligence ministry of Iran.
Dicho lo anterior, asumamos que el oficio cinematografico permite representaciones sociales que conforman una determinada impresion de la realidad mediante la presentacion de imagenes que se asumen como verdad refutable.
Constituye la realidad, y, por consiguiente, jamas es refutable.
But le impact of the combination of two factors - a gradual as posed to the frenetic approach )wards financial liberalization, and the prudent, and maybe even interventionist model of banking supervision and regulation - is refutable in insulating the banking system in emerging economies in Asia, from risks of le nature seen in recent months.