Wrong type inferenced for locally defined function
Consider the following example.
data A = A
deriving Show
data B = B
-- deriving Show is missing here.
main :: IO ()
main = do
let myPrint = putStrLn . show
myPrint A
myPrint B
The Show
instance for B
is missing and the most general type for myPrint
is Show a => a -> IO ()
.
Ghc, however, reports the following error:
file.hs:10:13: error:
• Couldn't match expected type ‘A’ with actual type ‘B’
• In the first argument of ‘myPrint’, namely ‘B’
In a stmt of a 'do' block: myPrint B
In the expression:
do { let myPrint = putStrLn . show;
myPrint A;
myPrint B }
This error won't help the programmer find the missing deriving Show
declaration. Instead, the error should read something like No instance for (Show B) arising from a use of ‘myPrint’
.
Weirdly, when I type this main
function into ghci, the error is 'correct'.
Trac metadata
Trac field | Value |
---|---|
Version | 8.0.1 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | Compiler |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | |
Operating system | |
Architecture |