meaning of generic type variable

1. generic type variable Also known as a "schematic type variable". Different occurrences of a generic type variable in a type expression may be instantiated to different types. Thus, in the expression let id x = x in id True, id 1 ids type is for all a: a -> a. The universal quantifier "for all a:" means that a is a generic type variable. For the two uses of id, a is instantiated to Bool and Int. Compare this with let id x = x in let f g = g True, g 1 in f id This looks similar but f has no legal Hindley-Milner type. If we say f :: a -> b -> b, b this would permit gs type to be any instance of a -> b rather than requiring it to be at least as general as a -> b. Furthermore, it constrains both instances of g to have the same result type whereas they do not. The type variables a and b in the above are implicitly quantified at the top level: f :: for all a: for all b: a -> b -> b, b so instantiating them removing the quantifiers can only be done once, at the top level. To correctly describe the type of f requires that they be locally quantified: f :: for all a: a -> for all b: b -> c, d which means that each time g is applied, a and b may be instantiated differently. fs actual argument must have a type at least as general as for all a: a -> for all b: b, and may not be some less general instance of this type. Type variables c and d are still implicitly quantified at the top level and, now that gs result type is a generic type variable, any types chosen for c and d are guaranteed to be instances of it. This type for f does not express the fact that b only needs to be at least as general as the types c and d. For example, if c and d were both Bool then any function of type for all a: a -> Bool would be a suitable argument to f but it would not match the above type for f. Genesia An expert system developed by Electricite de France and commercialised by STERIA Paris. genetic algorithm GA An evolutionary algorithm which generates each individual from some encoded form known as a "chromosome" or "genome". Chromosomes are combined or mutated to breed new individuals. "Crossover", the kind of recombination of chromosomes found in sexual reproduction in nature, is often also used in GAs. Here, an offsprings chromosome is created by joining segments choosen alternately from each of two parents chromosomes which are of fixed length. GAs are useful for multidimensional optimisation problems in which the chromosome can encode the values for the different variables being optimised. Illinois Genetic Algorithms Laboratory http://GAL4. GE. UIUC. EDU/illigal. home. html IlliGAL.

Related Words

generic type variable |

Developed & Maintained By Taraprasad.com