Static type system

Explicit or implicit declaration and inference

Many static type systems, such as those of C and Java, require type declarations: the programmer must explicitly associate each variable with a specific type. Others, such as Haskell's, perform type inference: the compiler draws conclusions about the types of variables based on how programmers use those variables. For example, given a function f(x,y) that adds x and y together, the compiler can infer that x and y must be numbers—since addition is only defined for numbers. Thus, any call to f elsewhere in the program that specifies a non-numeric type (such as a string or list) as an argument would signal an error.