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I've frequently heard claims that Haskell doesn't have variables; in particular, this answer claims that it doesn't, and it was upvoted at least nine times and accepted.
So does it have variables or not, and why?
This question also appears to apply ML, F#, OCaml, Erlang, Oz, Lava, and all SSA intermediate languages.
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We first note that variables in Haskell can only be assigned once, unlike in many imperative programming languages, where a variable can be overwritten with different values arbitrarily many times.
So, in particular, all those tasks which you use global variables for in C, can be achieved in haskell, but which solution depends what you are trying to do: If you have some globals which are constant, then just define them at the top level.
Expressions in Haskell are immutable. They cannot change after they are evaluated. Immutability makes refactoring super easy and code much easier to reason about. To “change” an object, most data structures provide methods taking the old object and creating a new copy.
You declare a variable; Haskell doesn't allow uninitialized variables, so you are required to supply a value in the declaration; There's no mutation, so the value given in the declaration will be the only value for that variable throughout its scope.
Haskell has immutable variables (variables in the math sense) by default:
foo x y = x + y * 2 By default variables are not mutable cells.
Haskell also has mutable cells though, but you enable them explicitly:
> import Data.IORef (newIORef, readIORef, writeIORef) > v <- newIORef 0 > readIORef v 0 > writeIORef v 7 > readIORef v 7 So, YES Haskell has true variables. But it does not use mutable variables by default.
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