Getting reading to solve this

.gitignore

@@ -1,2 +1,3 @@
 dist*
 
+.stack-work

Main.hs

@@ -1,4 +1,18 @@
 module Main where
 
+import Data.Set (Set)
+import qualified Data.Set as Set
+import qualified Data.Text.IO as TIO
+import qualified Data.Text as T
+import Data.Text.Read
+
 main :: IO ()
-main = putStrLn "Hello, Haskell!"
+main = do
+    inputs <- TIO.readFile "input.txt"
+    let inputs' = T.lines inputs
+    let xs = map decimal inputs'
+    let ys = map (2020-) xs
+    
+    let match = (Set.fromList xs) `Set.intersection` (Set.fromList ys)
+
+    putStrLn $ show $ Set.toList match

advent-of-code.cabal

@@ -0,0 +1,27 @@
+cabal-version: 1.12
+
+-- This file has been generated from package.yaml by hpack version 0.34.2.
+--
+-- see: https://github.com/sol/hpack
+
+name:           advent-of-code
+version:        0.1.0
+homepage:       https://git.bytes.zone/sehqlr/advent-of-code
+author:         Sam Hatfield <hey@samhatfield.me>
+maintainer:     Sam Hatfield <hey@samhatfield.me>
+license:        MIT
+build-type:     Simple
+
+source-repository head
+  type: git
+  location: https://git.bytes.zone/sehqlr/advent-of-code
+
+executable advent-of-code
+  main-is: Main.hs
+  other-modules:
+      Paths_advent_of_code
+  build-depends:
+      base
+    , containers
+    , text
+  default-language: Haskell2010

input.txt

@@ -0,0 +1,200 @@
+1765
+1742
+1756
+1688
+1973
+1684
+1711
+1728
+1603
+1674
+1850
+1836
+1719
+1937
+1970
+1770
+1954
+1848
+1885
+1851
+1474
+1801
+1769
+1904
+1906
+1739
+1717
+1830
+1985
+1930
+1791
+1977
+1713
+1787
+1773
+1672
+1750
+1931
+1807
+1762
+1835
+1736
+1979
+1923
+1782
+1797
+1822
+1903
+1729
+343
+1678
+1753
+1873
+1358
+1987
+1821
+1761
+1988
+1886
+1669
+857
+1894
+1404
+1909
+1789
+1752
+1882
+1969
+1892
+1701
+1956
+1839
+483
+1897
+1730
+1829
+1879
+1810
+1755
+1799
+1932
+1715
+1809
+418
+1896
+1691
+1749
+1922
+1631
+1780
+1734
+1859
+1695
+1548
+1948
+1997
+1921
+1994
+1369
+1364
+1764
+1697
+1833
+1239
+616
+1786
+1890
+677
+1867
+1705
+1993
+1925
+1774
+1732
+1686
+1847
+1911
+1841
+1962
+1907
+1919
+1725
+1687
+1236
+1864
+1855
+1928
+1941
+1709
+1683
+1676
+1889
+1982
+1595
+1735
+1857
+1731
+1816
+2003
+1724
+741
+1655
+1308
+1959
+1955
+1768
+1795
+1804
+1961
+1693
+1884
+1813
+1927
+1845
+1689
+1646
+1803
+2008
+1599
+1984
+1871
+1814
+1918
+1990
+1858
+1908
+1949
+1980
+1618
+1718
+1712
+1989
+1876
+1947
+1974
+1838
+1865
+1842
+1817
+680
+1986
+1812
+1895
+1991
+1644
+1877
+1880
+1792
+1800
+1899
+1806
+1699
+1685
+1793
+1647
+1429
+1751
+1722
+1887
+1968

package.yaml

@@ -0,0 +1,16 @@
+spec-version: 0.30.0
+name: advent-of-code
+version: 0.1.0
+homepage: "https://git.bytes.zone/sehqlr/advent-of-code"
+author: "Sam Hatfield <hey@samhatfield.me>"
+license: MIT
+build-type: Simple
+git: "https://git.bytes.zone/sehqlr/advent-of-code"
+
+dependencies:
+    - base
+    - containers
+    - text
+
+executable:
+    main: Main.hs

shell.nix

@@ -1,6 +1,8 @@
-(import ./default.nix {}).shellFor {
-  tools = {
-    cabal = "3.2.0.0";
-    hlint = "latest";
-  };
+{ ghc }:
+with (import <nixpkgs> { });
+
+haskell.lib.buildStackProject {
+  inherit ghc;
+  name = "adventEnv";
+  buildInputs = [ ];
 }

stack.yaml

@@ -64,3 +64,7 @@ packages:
 #
 # Allow a newer minor version of GHC than the snapshot specifies
 # compiler-check: newer-minor
+
+nix:
+    enable: true
+    shell-file: shell.nix

stack.yaml.lock

@@ -0,0 +1,12 @@
+# This file was autogenerated by Stack.
+# You should not edit this file by hand.
+# For more information, please see the documentation at:
+#   https://docs.haskellstack.org/en/stable/lock_files
+
+packages: []
+snapshots:
+- completed:
+    size: 524996
+    url: https://raw.githubusercontent.com/commercialhaskell/stackage-snapshots/master/lts/14/27.yaml
+    sha256: 7ea31a280c56bf36ff591a7397cc384d0dff622e7f9e4225b47d8980f019a0f0
+  original: lts-14.27