Haskell Thunks

A thunk is an unevaluated promise for a value in Haskell. It holds the work needed to compute the value until that value is forced.

The program defines a large sum but never uses it. Only the small result is printed.

main :: IO ()
main = let big = sum ([1..1000] :: [Int]) in print (1 + 1)

The product is a thunk until print asks for it. Printing it gives 12.

main :: IO ()
main = let x = 3 * 4 in print x

The same thunk is added to itself. Both uses see the value 10, so the total is 20.

main :: IO ()
main = let y = 2 * 5 in print (y + y)

Example

main :: IO ()
main = do
  let base = 4 * 5
  let doubled = base * 2
  print base
  print doubled

In this example, base is a thunk for 20. Doubled is built from base and is a thunk for 40. The two prints force each thunk in turn.

a thunk is invisible in ordinary code, so its name is the only sign of it.

Edit the code, then press Run.

Running is simulated · Haskell Playground

main :: IO ()
main = do
  let base = 4 * 5
  let doubled = base * 2
  print base
  print doubled

What is a thunk?

A finished value stored in memory A type of list An unevaluated promise for a value A function that prints a result