Differences

This shows you the differences between two versions of the page.

--- codesnippets:typeapplications [2021/04/23 19:12] – created f2b216
+++ codesnippets:typeapplications [2025/10/08 00:48] (current) – external edit 127.0.0.1
@@ Line 1: / Line 1: @@
 ====== {-# LANGUAGE TypeApplications #-} ======
+  * allows to specify types to apply
+  * example, without type application
+    * code:<code Haskell>
+module Main where
+main :: IO ()
+main = print (read "123")</code>
+    * compiles with error:<code>
+app\Main.hs:4:15: error:
+    * Ambiguous type variable `a0' arising from a use of `read'
+      prevents the constraint `(Read a0)' from being solved.
+      Probable fix: use a type annotation to specify what `a0' should be.
+      These potential instances exist:
+        instance Read Ordering -- Defined in `GHC.Read'
+        instance Read Integer -- Defined in `GHC.Read'
+        instance Read a => Read (Maybe a) -- Defined in `GHC.Read'
+        ...plus 22 others
+        ...plus 12 instances involving out-of-scope types
+        (use -fprint-potential-instances to see them all)
+    * In the first argument of `print', namely `(read "123")'
+      In the expression: print (read "123")
+      In an equation for `main': main = print (read "123")
+  |
+| main = print (read "123")
+  |               ^^^^^^^^^^
+</code>
+  * example, without type application
+    * code:<code Haskell>
+module Main where
+main :: IO ()
+main = print ((read "123") :: Integer)
+</code>
+  * example, with type application
+    * code:<code Haskell>
+{-# LANGUAGE TypeApplications  #-}
+module Main where
+main :: IO ()
+main = print (read @Integer "123")
+</code>
+===== ✎ =====
+~~DISCUSSION~~