3 ms·

Prelude> let x = [1, 2, 3] Prelude> x [1,2,3] Prelude> x // [(2,42)] <interactive>:5:3: Not in scope: `//' Perhaps you meant on

• mej10 9y ago
  I understand the source of your question -- not all languages provide arrays in the included-by-default namespace. Python for example requires you to import an array module. These are part of the standard library in both cases. This makes it a little harder to get started but considering the kinds of things programmers put up with on a daily basis is extremely mild.
• berdario 9y ago
  The Prelude is just what's visible in the default namespace, it's not the whole language Data.Array and // are also described in the Haskell 2010 report https://www.haskell.org/onlinereport/haskell2010/haskellch14.html https://www.haskell.org/onlinereport/haskell2010/haskellch14...
• tome 9y ago
  Further to the other answers given, `[1, 2, 3]` is not an array, but rather a list.
• mitchty 9y ago
  The prelude isn't everything in Haskell. Just like you need to import sys or do import re in python to get at things less used here too you have to do the same. λ :m Data.Array λ let x = array (1,3) [(1,1), (2,2), (3,3)] x :: (Num e, Num i, Ix i) => Array i e λ x array (1,3) [(1,1),(2,2),(3,3)] it :: (Ix i, Num i, Num e) => Array i e λ x // [(2,42)] array (1,3) [(1,1),(2,42),(3,3)] it :: (Num i, Num e, Ix i) => Array i e λ x array (1,3) [(1,1),(2,2),(3,3)] it :: (Ix i, Num i, Num e) => Array i e Note however that this isn't mutating x directly, to do mutation would require state or a monad.