Haskell Language Infix operators Prelude
Example
Logical
&& is logical AND, || is logical OR.
== is equality, /= non-equality, < / <= lesser and > / >= greater operators.
Arithmetic operators
The numerical operators +, - and / behave largely as you'd expect. (Division works only on fractional numbers to avoid rounding issues – integer division must be done with quot or div). More unusual are Haskell's three exponentiation operators:
-
^takes a base of any number type to a non-negative, integral power. This works simply by (fast) iterated multiplication. E.g.4^5 ≡ (4*4)*(4*4)*4 -
^^does the same in the positive case, but also works for negative exponents. E.g.3^^(-2) ≡ 1 / (2*2)Unlike
^, this requires a fractional base type (i.e.4^^5 :: Intwill not work, only4^5 :: Intor4^^5 :: Rational). -
**implements real-number exponentiation. This works for very general arguments, but is more computionally expensive than^or^^, and generally incurs small floating-point errors.2**pi ≡ exp (pi * log 2)
Lists
There are two concatenation operators:
-
:(pronounced cons) prepends a single argument before a list. This operator is actually a constructor and can thus also be used to pattern match (“inverse construct”) a list. -
++concatenates entire lists.[1,2] ++ [3,4] ≡ 1 : 2 : [3,4] ≡ 1 : [2,3,4] ≡ [1,2,3,4]
!! is an indexing operator.
[0, 10, 20, 30, 40] !! 3 ≡ 30
Note that indexing lists is inefficient (complexity O(n) instead of O(1) for arrays or O(log n) for maps); it's generally preferred in Haskell to deconstruct lists by folding ot pattern matching instead of indexing.
Control flow
-
$is a function application operator.f $ x ≡ f x ≡ f(x) -- disapproved styleThis operator is mostly used to avoid parentheses. It also has a strict version
$!, which forces the argument to be evaluated before applying the function. -
.composes functions.(f . g) x ≡ f (g x) ≡ f $ g x -
>>sequences monadic actions. E.g.writeFile "foo.txt" "bla" >> putStrLn "Done."will first write to a file, then print a message to the screen. -
>>=does the same, while also accepting an argument to be passed from the first action to the following.readLn >>= \x -> print (x^2)will wait for the user to input a number, then output the square of that number to the screen.
