Merge pull request #346 from haskellfoundation/GHC-91938

Document GHC-91938

Author: david-christiansen

message-index/messages/GHC-91938/example1/after/Main.hs

@@ -0,0 +1,2 @@
+f Nothing x = x
+f (Just y) x = y + x

message-index/messages/GHC-91938/example1/before/Main.hs

@@ -0,0 +1,2 @@
+f Nothing = \x -> x
+f (Just y) x = y + x

message-index/messages/GHC-91938/example1/index.md

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+---
+title: Varying numbers of arguments
+---
+
+In this example, the function `f` has type `Num a => Maybe a -> a -> a`. Even though each case of the definition can have this type on its own, one of them has a single argument to the left of the `=` and a lambda on the right-hand side, and the other has two arguments to the left of the `=`. In Haskell, each defining equation must have the same number of arguments prior to the `=`.
+
+The program was fixed by moving the lambda's argument to the left of the `=`.
+
+## Error Message
+
+```
+Main.hs:1:1: error: [GHC-91938]
+    Equations for ‘f’ have different numbers of arguments
+      Prog.hs:2:1-19
+      Prog.hs:3:1-20
+  |
+2 | f Nothing = \x -> x
+  | ^^^^^^^^^^^^^^^^^^^...
+```
+

message-index/messages/GHC-91938/index.md

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+---
+title: Equations have different number of arguments
+summary: Each equation in a function definition must have the same number of arguments.
+severity: error
+introduced: 9.6.1
+---
+
+Haskell functions are defined using equations, with the function name and patterns on the left side of the `=` and expressions on the right side. When the function is applied, the patterns in the equations are checked from top to bottom, with the first matching equation being chosen.
+
+Each equation that defines a function must have the same number of patterns to the left of the `=`.
+
+