Haskell what does the ' symbol do?

Question

As the title states, I see pieces of code online where the variables/functions have ' next to it, what does this do/mean? ex:

function :: [a] -> [a]
function ...

function' :: ....

Answer 1

The notation comes from mathematics. It is read x prime. In pretty much any math manual you can find something like let x be a number and x' be the projection of ... (math stuff).

Why not using another convention? well, in mathematics It makes a lot of sense because It can be very pedagogical... In programming we aren't used to this convention so I don't see the point of using it, but I am not against it neither.

Just to give you an example of its use in mathematics so you can understand why It is used in Haskell. Below, the same triangle concept but one using prime convention and other not using it. It is pretty clear in the first picture that pairs (A, A'), (B, B'), ... are related by one being the vertex and the prime version being the midpoint of the oposite edge. Whereas in the second example, you just have to remember that A is the midpoint of the oposite edge of vertex P. First is easier and more pedagogical:

Answer 2

As the other answers said, function' is just another variable name. So,

 don'tUse :: Int -> IO ()
 don'tUse won'tBe''used'' = return ()

is just like

 dontUse :: Int -> IO ()
 dontUse wontBeUsed = return ()

with slightly different names. The only requirement is that the name starts with a lowercase-letter or underscore, after that you can have as many single-quote characters as you want.

Prelude> let _' = 1
Prelude> let _'' = 2
Prelude> let _''''''''' = 9
Prelude> _' + _'' * _'''''''''
19

...Of course it's not necessarily a good idea to name variables like that; normally such prime-names are used when making a slightly different version of an already named thing. For example, foldl and foldl' are functions with the same signature that do essentially the same thing, only with different strictness (which often affects performance memory usage and whether infinite inputs are allowed, but not the actual results).

That said, to the question

Haskell what does the ' symbol do?

– the ' symbol does in fact do various other things as well, but only when it appears not as a non-leading character in a name.

• 'a' is a character literal.
• 'Foo is a constructor used on the type level. See DataKinds.
• 'bar and ''Baz are quoted names. See TemplateHaskell.