Document GHC-01239 (closes #288)

Author: jhrcek

message-index/messages/GHC-01239/example1/after/IfInFunAppExpr.hs

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+{-# LANGUAGE BlockArguments #-}
+module IfInFunAppExpr where
+
+-- Requires BlockArguments language extension to work
+example :: Int
+example = length if True then [1] else
+
+-- Works without BlockArguments
+example :: Int
+example = length (if True then [1] else [])
+
+-- Works without BlockArguments
+example2 :: Int
+example2 = length $ if True then [1] else []

message-index/messages/GHC-01239/example1/before/IfInFunAppExpr.hs

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+module IfInFunAppExpr where
+
+example :: Int
+example = length if True then [1] else []

message-index/messages/GHC-01239/example1/index.md

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+---
+title: Unexpected if expression in function application
+---
+
+To pass if-then-else expressions as function argument we either have to surround it in parentheses,
+use function application operator `($)` or enable [`BlockArguments`](https://ghc.gitlab.haskell.org/ghc/doc/users_guide/exts/block_arguments.html#extension-BlockArguments) extension.
+
+## Error Message
+```
+IfInFunAppExpr.hs:4:18: error:
+    Unexpected if expression in function application:
+        if True then [1] else []
+    Suggested fixes:
+      • Use parentheses.
+      • Perhaps you intended to use BlockArguments
+  |
+4 | example = length if True then [1] else []
+  |                  ^^^^^^^^^^^^^^^^^^^^^^^^
+```

message-index/messages/GHC-01239/index.md

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+---
+title: Unexpected if expression in function application
+summary: If expression used as function argument
+severity: error
+introduced: TODO
+---
+
+Unlike in many other languages, in Haskell the if-then-else construct is an expression, which means it returns a value that can be processed further.
+
+```language-haskell
+ageMessage :: Int -> String 
+ageMessage age = if age < 18 then "You are too young to enter" else "Welcome to the club"
+
+putStrLn (ageMessage 10) -- You are too young to enter
+putStrLn (ageMessage 20) -- Welcome to the club
+```
+
+Thinking of if-then-else expression as a value, we might want to pass it as an input to a function.
+If you do that you can run into the following error.