Why does not get evaluate?

Asked Sep 27 '17 at 13:37

Active Sep 27 '17 at 13:37

Viewed 66 times

I have following code snippet:

*Main Lib> let myList = [1, 2, id 1] :: [Integer]
*Main Lib> let myList' = myList ++ undefined
*Main Lib> :sprint myList'
myList' = _

Why does the myList does not get evaluated?

And this too:

*Main Lib> x = 45
*Main Lib> :sprint x
x = _

asked Sep 27 '17 at 13:37

softshipper

1

Because nothing is evaluated unless it absolutely has to in Haskell? – Willem Van Onsem Sep 27 '17 at 13:49
1

See [here](https://stackoverflow.com/a/42602657/67579) – Willem Van Onsem Sep 27 '17 at 13:50
1

Why do you think it should get evaluated? – augustss Sep 27 '17 at 13:58
@augustss: I think the OP means that `x = 45` has not much to evaluate. But because `ghci` uses the `MonomorphismRestriction`, it does not pick a type for `x` (so no `Int`, `Integer`, etc.) – Willem Van Onsem Sep 27 '17 at 14:05
Let's consider the first example. When I write `*Main Lib> myList ++ undefined` then it is an application and it gets evaluated. When I write `let myList' = myList ++ undefined` is a lambda abstraction, that why it does not get evaluated, right? – softshipper Sep 27 '17 at 14:14
2

Because if you write `myList ++ undefined`, you have implicitly written `print (myList ++ undefined)`. So you force Haskell to generate textual output, and this requires some evaluation. – Willem Van Onsem Sep 27 '17 at 14:26
and what about ` myList'`? – softshipper Sep 27 '17 at 14:37
2

a `let` statement is never evaluated eagerly. Only if it needs to be, like anything in Haskell. – Willem Van Onsem Sep 27 '17 at 14:44
@WillemVanOnsem thanks for the link. – Will Ness Sep 27 '17 at 15:21

0 Answers0