Case expression included in pattern [GHC-53786]
A pattern (e.g. in a function definition or a pattern match) may not contain case ... of ... syntax.
See here for an overview of the syntax and semantics of patterns and pattern matching in Haskell.
Examples
Case expression in case pattern
The pattern being matched upon in the case expression case x of ... contains another case expression.
Error Message
CaseExprInCasePattern.hs:5:3: error: [GHC-53786]
(case ... of ...)-syntax in pattern
|
5 | case y of
| ^^^^^^^^^...CaseExprInCasePattern.hs
Before
module CaseExprInFunctionDef where
f :: a -> b -> Int
f x y = case x of
case y of
_ -> 1
After
module CaseExprInFunctionDef where
f :: a -> b -> Int
f x y = case x of
_ -> case y of
_ -> 1
Case expression in do-block
The expression in one branch of the case expression is a do-block that’s missing a do. This triggers a corner case in the grammar of Haskell, and the case-expression is interpreted as the pattern part of a bind statement in the top-level do-block.
More details are available on the GHC issue tracker, issue #984.
Error Message
CaseExprInDoBlock.hs:5:3: error: [GHC-53786]
(case ... of ...)-syntax in pattern
|
5 | case () of
| ^^^^^^^^^^...
CaseExprInDoBlock.hs
Before
module CaseExprInDoBlock where
x :: [a]
x = do
case () of
() ->
a <- []
pure a
After
module CaseExprInDoBlock where
x :: [a]
x = do
case () of
() -> do
a <- []
pure a
Case expression in function argument pattern
The pattern on the left-hand side of the function definition contains a case ... of ... expression.
Error Message
CaseExprInFunctionDef.hs:3:4: error: [GHC-53786]
(case ... of ...)-syntax in pattern
|
3 | f (case x of { Nothing -> 0; Just a -> a }) = a
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^CaseExprInFunctionDef.hs
Before
module CaseExprInFunctionDef where
f (case x of { Nothing -> 0; Just a -> a }) = a
After
module CaseExprInFunctionDef where
f Nothing = 0
f (Just a) = a