ghc

问题 I'm trying to build the double-conversion package from Hackage. It builds OK but ghci can't load it. The complaint is: Loading package double-conversion-0.2.0.0 ... linking ... <interactive>: /home/me/.cabal/lib/double-conversion-0.2.0.0/ghc-7.0.4/HSdouble-conversion-0.2.0.0.o: unknown symbol `_ZNK17double_conversion6Double5valueEv' The nm output shows that the symbol is weak : nm /home/me/.cabal/lib/double-conversion-0.2.0.0/ghc-7.0.4/HSdouble-conversion-0.2.0.0.o \ | grep _ZNK17double

问题 I'm getting parse error on input ‘where’ when trying the following example in GHC 7.10.2: {-# LANGUAGE TypeFamilies #-} type family F a :: * type instance where F (Maybe Int) = Int F (Maybe Bool) = Bool F (Maybe a) = String Same question was asked two years ago about GHC 7.4.2. I used type families fine with GHC 7.6.* (can't remember last digit) this year. Is it a problem with GHC 7.10.2? According to the User's Guide, type families are available. My GHC 7.10.2 and cabal 1.22.6.0 where

问题 Heterogeneous lists are one of the examples given for the new dependent type facility of ghc 7.6: data HList :: [*] -> * where HNil :: HList '[] HCons:: a -> HList t -> HList (a ': t) The example list "li" compiles fine: li = HCons "Int: " (HCons 234 (HCons "Integer: " (HCons 129877645 HNil))) Obviously we would like HList to be in the Show class, but I can only come up with the following working class instantiation that uses mutually recursive constraints (superclasses): instance Show (HList

问题 This C code can conceptually be described as creating a new array identical to an input array but with 1 as the first element: int* retire_and_update(int* arr) { arr[0] = 1; return arr; } This is a pure function (wink wink nudge nudge) so long as no further references are made to the input array and its elements. The C type system won't enforce that for us but it seems enforceable in principle. The code that gcc generates is simple and efficient: retire_and_update: movq %rdi, %rax movl $1, (

问题 I recently discovered -XTypeApplications , which allows me to write abbreviate my code. Consider the following: {-# LANGUAGE AllowAmbiguousTypes, DataKinds, KindSignatures, RankNTypes, ScopedTypeVariables, TypeApplications #-} import Data.Tagged import Data.Proxy class Foo (a :: Bool) where foo :: Tagged a Int instance Foo 'True where foo = 3 instance Foo 'False where foo = 4 instead of writing proxy foo (Proxy::Proxy 'True) , I can now write untag $ foo @'True , which saves me a lot of Proxy

问题 I've got some unused functionality in my codebase, but it's hard to identify. The code has evolved over the last year as I explore its problem space and possible solutions. What I'm needing to do is find that unused code so I can get rid of it. I'm happy if it deals with the problem on an exportable name basis.GHC has warnings that deal with non-exported unused code. Any tools specific to this task would be of interest. However, I'm curious about a comprehensive cross referencing tool. I can

问题 This is an embarrassingly simple problem, but its solution yet eludes me. As the title indicates, I simply want to specify to GHC the location of all my source files. This should be simple; the GHC user guide: -i dirs This flag appends a colon-separated list of dirs to the search path. So, I tried the following invocations: ghc -isrc/ -v -outputdir build/ --make -Wall Main.hs ghc -isrc/: -v -outputdir build/ --make -Wall Main.hs ghc -i:src/: -v -outputdir build/ --make -Wall Main.hs ghc -i

问题 This is an embarrassingly simple problem, but its solution yet eludes me. As the title indicates, I simply want to specify to GHC the location of all my source files. This should be simple; the GHC user guide: -i dirs This flag appends a colon-separated list of dirs to the search path. So, I tried the following invocations: ghc -isrc/ -v -outputdir build/ --make -Wall Main.hs ghc -isrc/: -v -outputdir build/ --make -Wall Main.hs ghc -i:src/: -v -outputdir build/ --make -Wall Main.hs ghc -i

问题 I'd like to write a Haskell program that uses GADTs interactively on a platform not supported by GHCi (namely, GNU/Linux on mipsel). The problem is, the construct that can be used to define a GADT in GHC, for example: data Term a where Lit :: Int -> Term Int Pair :: Term a -> Term b -> Term (a,b) ... doesn't seem working on Hugs. Can't GADTs really be defined in Hugs? My TA at a Haskell class said it was possible in Hugs, but he seemed unsure. If not, can GADT be encoded by using other syntax

问题 I am trying to install HTF. However after I cabal install HTF I get this: Resolving dependencies... Configuring HTF-0.10.0.7... Warning: This package indirectly depends on multiple versions of the same package. This is highly likely to cause a compile failure. package regex-base-0.93.2 requires mtl-2.0.1.0 package aeson-0.6.0.2 requires mtl-2.1.2 package HTF-0.10.0.7 requires mtl-2.1.2 package mtl-2.0.1.0 requires transformers-0.2.2.0 package transformers-base-0.4.1 requires transformers-0.3