GHC only provides tuples up to size 62, and derived tuple instances (for
Eq, Ord, etc) up to size 15.
GHC can warn about non-exhaustive or overlapping patterns, and usually does so correctly.
But not always. It gets confused by string patterns, and by guards, and can then
emit bogus warnings. The entire overlap-check code needs an overhaul really.
Dangers with multiple Main modules.
GHC does not insist that module Main lives in a file called Main.hs.
This is useful if you want multiple versions of Main. But there's a danger: when
compiling module Main (regardless of what file it comes from), GHC looks for
the interface Main.hi; it uses this to get version information from the last
time it recompiled Main. The trouble is that this Main.hi
may not correspond to the source file being compiled.
Solution: remove Main.hi first. A better solution would be for GHC to
record the source-file filename in the interface file, or even an MD5 checksum.
GHCi does not respect the default declaration in the module whose
scope you are in. Instead, for expressions typed at the command line, you always
get the default default-type behaviour; that is, default(Int,Double).
It would be better for GHCi to record what the default settings in each module are, and
use those of the 'current' module (whatever that is).
GHCi does not keep careful track of what instance declarations are 'in scope' if they
come from other packages.
Instead, all instance declarations that GHC has seen in other packages are all in scope
everywhere, whether or not the module from that package is used by the command-line expression.
GHC's inliner can be persuaded into non-termination using the standard way to encode
recursion via a data type:
data U = MkU (U -> Bool)
russel :: U -> Bool
russel u@(MkU p) = not $ p u
x :: Bool
x = russel (MkU russel)
We have never found another program, other than this contrived one, that makes GHC
diverge, and fixing the problem would impose an extra overhead on every compilation. So the
bug remains un-fixed. There is more background in
Secrets of the GHC inliner.