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Given a function, for example a function f of type Int -> Bool, the effect of calling the function on an Int must be nothing other than returning a Bool.

For example, f cannot read a file, write to command line, or start a thread, as a side effect of being called.

Of course, it is possible to do all these things, but requires a change to the type:

add1AndPrint :: Int -> IO Bool
add1AndPrint x = (print x) >> (return (x > 1)) -- (1)!
  1. Fewer brackets are fine (add1AndPrint x = print x >> return (x > 1)), and are here just for clarity.

This applies not just to functions, but all values:

repl example
> let boolean = print "hello" >> return True
> :t boolean
boolean :: IO Bool -- (1)!
> boolean
  1. boolean cannot have type Bool - because its definition calls print, it has to mark in its type the fact that it involves the operation of printing.

The benefit of purity

Because of purity, a function will give the same answer no matter where or when it is called, as long as the input is the same. This makes parallelism easy, often almost trivial:

main :: IO ()
main = hspec $ do
    ... tests ...
main :: IO ()
main = hspec $ parallel $ do
    ... tests ...

Purity also lends itself to modular code, and easy refactoring. Consider:

graphicalUserInterface = runWith complexFunction
        complexFunction :: UserInput -> Picture
        complexFunction = ...

        runWith = ... -- e.g., a handler function

Suppose we want to replace complexFunction with simpleFunction, which also has the type UserInput -> Picture.

Because Haskell is pure (but see caveats) and so complexFunction is not creating/mutating global variables, or opening or closing files, we can be confident that there will be no unexpected implications of making the change, such as a subtle error with changed variable assignment, when runWith takes complexFunction as input.

Equational reasoning

Because of purity, you may always replace a function call in Haskell with its resulting value. For instance, if the value of positionOfWhiteKing chessboard is "a4", then this

exampleProgram = someFunction (positionOfWhiteKing chessboard)

is equivalent to

exampleProgram = someFunction "a4"


Use this fact to understand complex programs, by substituting complex expressions for their values:

data Piece = Bishop | Rook | King
take 2 [Bishop, Rook, Bishop]

To work out what this does, we consult the definition of take (shown here with some aesthetic simplifications for clarity):

take 0 ls = []
take _ [] = []
take n (firstElem : rest) = firstElem : take (n-1) rest

Following this definition, we replace take 2 [1,2,3] (or more explicitly, #1hs take 2 (1 : [2,3])) with the pattern that it matches:

    take 2 (Bishop : [Rook, Bishop]) 
    = Bishop : take (2-1) [Rook, Bishop] 
    = Bishop : take 1 (Rook : [Bishop])

We can continue in this vein, repeatedly consulting the definition of take:

    = Bishop : take 1 (Rook : [Bishop])
    = Bishop : (Rook : take (1 - 1) [Bishop])
    = Bishop : (Rook : take 0 [Bishop]) 
    = Bishop : (Rook : [])
    = [Bishop, Rook]

This technique is always applicable, no mater how complex the program.


A function is total if it returns a result for any possible input. For example, head is not total:

repl example
> head [1,2]
> head []
*** Exception: Prelude.head: empty list

In Haskell, non-total (aka partial) functions are permitted, although they are discouraged. Functions may be partial by throwing an runtime error on some inputs (like head), or by running indefinitely, (like last [1..]). Haskell will generally warn you about the first kind, but not the second, since it is harder to detect.


Haskell allows a backdoor, mainly useful for debugging.

This is the ability for functions to throw an "unsafe" error:

repl example
let x = undefined
> :t x
x :: a
> x
"*** Exception: Prelude.undefined..."

undefined has the type forall a. a, so it can appear anywhere in a program and assume the correct type (see here for more details on how universal quantification works).

As such, it is useful as a "to do" marker (see type driven development).