Case comprehensions
One could argue that the notation
<-
is misleading, suggesting the idea of drawn from as in a list comprehension. But it is very nice to reuse precisely the list comprehension syntax.
- Pattern guards and Transformational Patterns, Martin Erwig and Simon Peyton Jones.
Reusing just that part of the list-comprehension syntax seems to work for multi-part function definitions:
clunky env var1 var2 | Just val1 <- lookup env var1,
Just val2 <- lookup env var2 = val1 + val2
| otherwise = var1 + var2
but when it's used in case expressions:
-- ghc-8.6.5/compiler/ghci/RtClosureInspect.hs, lines 1099-1107
check2 (_, rtti_ty) (_, old_ty)
| Just (_, rttis) <- tcSplitTyConApp_maybe rtti_ty
= case () of
_ | Just (_,olds) <- tcSplitTyConApp_maybe old_ty
-> and$ zipWith check2 (map quantifyType rttis) (map quantifyType olds)
_ | Just _ <- splitAppTy_maybe old_ty
-> isMonomorphicOnNonPhantomArgs rtti_ty
_ -> True
| otherwise = True
it looks somewhat out-of-place, unless:
map f xs = [ x <- xs -> f x ]
is considered a suitable replacement for:
map f xs = [ f x | x <- xs ]
Could the GHC example benefit from some more list-comprehension syntax? Let's borrow a new reserved word:
check2 (_, rtti_ty) (_, old_ty)
| Just (_, rttis) <- tcSplitTyConApp_maybe rtti_ty
= cases
and $ zipWith check2 (map quantifyType rttis) (map quantifyType olds) |
Just (_,olds) <- tcSplitTyConApp_maybe old_ty
isMonomorphicOnNonPhantomArgs rtti_ty |
Just _ <- splitAppTy_maybe old_ty
True | otherwise
| otherwise = True
...well, at least the unit-case workaround is gone. With a little refactoring:
check2 (_, rtti_ty) (_, old_ty)
| Just (_, rttis) <- tcSplitTyConApp_maybe rtti_ty
= case () of
_ | Just (_,olds) <- tcSplitTyConApp_maybe old_ty
-> and $ zipWith qcheck2 rttis olds
_ | Just _ <- splitAppTy_maybe old_ty
-> isMonomorphicOnNonPhantomArgs rtti_ty
_ -> True
| otherwise = True
qcheck2 rtti old = check2 (quantifyType rtti) (quantifyType old)
and the alternate syntax:
check2 (_, rtti_ty) (_, old_ty)
| Just (_, rttis) <- tcSplitTyConApp_maybe rtti_ty
= cases
and $ zipWith qcheck2 rttis olds | Just (_,olds) <- tcSplitTyConApp_maybe old_ty
isMonomorphicOnNonPhantomArgs rtti_ty | Just _ <- splitAppTy_maybe old_ty
True | otherwise
| otherwise = True
qcheck2 rtti old = check2 (quantifyType rtti) (quantifyType old)
this particular example is further improved, unless:
filter p xs = [ x <- xs, p x -> x ]
is considered a reasonable substitute for:
filter p xs = [ x | x <- xs, p x ]
Still not convinced? Here's some other examples:
-- ghc-8.6.5/compiler/codeGen/StgCmmMonad.hs, lines 819-827
let
(test, then_, else_, likely') = case likely of
Just False | Just e' <- maybeInvertCmmExpr e
-> (e', fbranch, tbranch, Just True)
_ -> (e, tbranch, fbranch, likely)
or:
let
(test, then_, else_, likely') =
cases
(e', fbranch, tbranch, Just True) | Just False <- likely,
Just e' <- maybeInvertCmmExpr e
(e, tbranch, fbranch, likely) | otherwise
-- ghc-8.6.5/compiler/basicTypes/Demand.hs, lines 927-934
splitProdDmd_maybe (JD { sd = s, ud = u })
= case (s,u) of
(Str _ (SProd sx), Use _ u) | Just ux <- splitUseProdDmd (length sx) u
-> Just (mkJointDmds sx ux)
(Str _ s, Use _ (UProd ux)) | Just sx <- splitStrProdDmd (length ux) s
-> Just (mkJointDmds sx ux)
(Lazy, Use _ (UProd ux)) -> Just (mkJointDmds (replicate (length ux) Lazy) ux)
_ -> Nothing
or:
splitProdDmd_maybe (JD { sd = s, ud = u })
= cases
Just (mkJointDmds sx ux) | Str _ (SProd sx) <- s, Use _ u <- u,
Just ux <- splitUseProdDmd (length sx) u
Just (mkJointDmds sx ux) | Use _ (UProd ux) <- u, Str _ s <- s,
Just sx <- splitStrProdDmd (length ux) s
Just (mkJointDmds (replicate (length ux) Lazy) ux) |
Use _ (UProd ux) <- u, Lazy <- s
Nothing | otherwise
(note the reordering of matches against s
and u
.)
-- ghc-8.6.5/compiler/specialise/SpecConstr.hs, lines 2209-2215
mb_scrut dc = case arg_occ of
ScrutOcc bs | Just occs <- lookupUFM bs dc
-> Just (occs)
_other | sc_force env || sc_keen env
-> Just (repeat UnkOcc)
| otherwise
-> Nothing
or:
mb_scrut dc =
cases
Just (occs) | ScrutOcc bs <- arg_occ, Just occs <- lookupUFM bs dc
Just (repeat UnkOcc) | sc_force env || sc_keen env
Nothing | otherwise
One more example:
concatMap f xs = [ x <- xs, y <- f x -> y ]
or:
concatMap f xs = [ y | x <- xs, y <- f x ]
Atravers 01:37, 10 July 2020 (UTC)