# Variable

### From HaskellWiki

In Haskell, a variable is a name for some valid expression. The word "variable" as applied to Haskell variables is misleading, since a given variable's value never varies during a program's runtime. Instead, the concept is much closer to the mathematical sense of the word, where one might use *p**i* to stand for 3.14159..., or *X*^{2} to represent the formula to compute to test for goodness of fit. These values do not generally change in the middle of performing a mathematical calculation or proof.

=

phi = 1.618

let

where

Another sense in which "variable" is used in Haskell is as formal parameters to a function. For example:

add x y = x + y

*x* and *y* are formal parameters for the *add* function. This corresponds to the notion of "variable" in the lambda calculus.