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The following code outputs "A".

import Data.Word

x :: Word
x = 10

y :: Word
y = 11

test = case x - y of
         -1 -> "A"
         _  -> "B"
main = putStrLn $ show test

However, adding a new case that isn't matched causes the output to change:

import Data.Word

x :: Word
x = 10

y :: Word
y = 11

test = case x - y of
         5 -> "C"
         -1 -> "A"
         _  -> "B"
main = putStrLn $ show test

With the extra '5 -> "C"' line in the case, the output is "B".

It gets weirder - interpreted code actually continues to match the -1 case. With the second example in a file 'T.hs', here is a GHCi session:

Prelude Main> :l *T.hs
[1 of 1] Compiling Main             ( T.hs, interpreted )
Ok, modules loaded: Main.
*Main> main
"A"
*Main> :l T.hs
Ok, modules loaded: Main.
Prelude Main> main
"B"
Prelude Main>

These examples suggest to me at least 3 improvements:

• Issue a warning when a negative number is used in a case statement in unsigned context
• Interpreted code should give the same result as compiled code
• Adding a case that isn't matched shouldn't change which other patterns get matched

Trac metadata

Trac field	Value
Version	7.8.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
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CC
Operating system	Windows
Architecture