ArrowLoop is more lazy

.gitignore

@@ -1,4 +1,5 @@
 .stack-work
+stack.yaml.lock
 dist
 cabal-dev
 *.sw[a-z]

src/Control/Varying/Core.hs

@@ -64,10 +64,12 @@ import           Prelude                    hiding (id, (.))
 -- | A continuously varying value, with effects.
 -- It's a kind of <https://en.wikipedia.org/wiki/Mealy_machine Mealy machine>
 -- (an automaton).
-newtype VarT m a b = VarT { runVarT :: a -> m (b, VarT m a b) }
-                            -- ^ Run a @VarT@ computation with an input value of
-                            -- type 'a', yielding a step - a value of type 'b'
-                            -- and a new computation for yielding the next step.
+newtype VarT m a b
+  = VarT
+  { runVarT :: a -> m (b, VarT m a b) }
+  -- ^ Run a @VarT@ computation with an input value of
+  -- type 'a', yielding a step - a value of type 'b'
+  -- and a new computation for yielding the next step.
 
 
 -- | A var parameterized with Identity that takes input of type @a@
@@ -175,8 +177,21 @@ instance Monad m => ArrowApply (VarT m) where
     (c, _) <- runVarT v b
     return (c, app)
 
+-- | Inputs can depend on outputs as long as no time-travel is required.
+--
+-- This isn't the best example but it does make a good test case:
+-- >>> :{
+-- let
+--   testVar :: VarT IO Double (Maybe Double)
+--   testVar = proc val -> do
+--     rec _ <- returnA -< 0.5
+--     returnA -< Just 5.0
+-- in
+--   testVarOver testVar [5.0]
+-- >>> :}
+-- Just 5.0
 instance MonadFix m => ArrowLoop (VarT m) where
-  loop vmbdcd = VarT $ \b -> fmap fst $ mfix $ \(_, d) -> do
+  loop vmbdcd = VarT $ \b -> fmap fst $ mfix $ \(~(_, d)) -> do
     ((c1, d1), vmbdcd1) <- runVarT vmbdcd (b, d)
     return ((c1, loop vmbdcd1), d1)
 

stack.yaml

@@ -2,7 +2,7 @@
 #
 # Some commonly used options have been documented as comments in this file.
 # For advanced use and comprehensive documentation of the format, please see:
-# http://docs.haskellstack.org/en/stable/yaml_configuration/
+# https://docs.haskellstack.org/en/stable/yaml_configuration/
 
 # Resolver to choose a 'specific' stackage snapshot or a compiler version.
 # A snapshot resolver dictates the compiler version and the set of packages
@@ -11,37 +11,56 @@
 # resolver: lts-3.5
 # resolver: nightly-2015-09-21
 # resolver: ghc-7.10.2
-# resolver: ghcjs-0.1.0_ghc-7.10.2
-# resolver:
-#  name: custom-snapshot
-#  location: "./custom-snapshot.yaml"
-resolver: lts-8.16
+#
+# The location of a snapshot can be provided as a file or url. Stack assumes
+# a snapshot provided as a file might change, whereas a url resource does not.
+#
+# resolver: ./custom-snapshot.yaml
+# resolver: https://example.com/snapshots/2018-01-01.yaml
+resolver: lts-14.5
 
 # User packages to be built.
 # Various formats can be used as shown in the example below.
 #
 # packages:
 # - some-directory
 # - https://example.com/foo/bar/baz-0.0.2.tar.gz
-# - location:
-#    git: https://github.com/commercialhaskell/stack.git
-#    commit: e7b331f14bcffb8367cd58fbfc8b40ec7642100a
-# - location: https://github.com/commercialhaskell/stack/commit/e7b331f14bcffb8367cd58fbfc8b40ec7642100a
-#   extra-dep: true
-#  subdirs:
-#  - auto-update
-#  - wai
-#
-# A package marked 'extra-dep: true' will only be built if demanded by a
-# non-dependency (i.e. a user package), and its test suites and benchmarks
-# will not be run. This is useful for tweaking upstream packages.
+#   subdirs:
+#   - auto-update
+#   - wai
 packages:
-- '.'
-# Dependency packages to be pulled from upstream that are not in the resolver
-# (e.g., acme-missiles-0.3)
-extra-deps: []
+- .
+# Dependency packages to be pulled from upstream that are not in the resolver.
+# These entries can reference officially published versions as well as
+# forks / in-progress versions pinned to a git hash. For example:
+#
+# extra-deps:
+# - acme-missiles-0.3
+# - git: https://github.com/commercialhaskell/stack.git
+#   commit: e7b331f14bcffb8367cd58fbfc8b40ec7642100a
+#
+# extra-deps: []
 
-flags: {}
+# Override default flag values for local packages and extra-deps
+# flags: {}
 
 # Extra package databases containing global packages
-extra-package-dbs: []
+# extra-package-dbs: []
+
+# Control whether we use the GHC we find on the path
+# system-ghc: true
+#
+# Require a specific version of stack, using version ranges
+# require-stack-version: -any # Default
+# require-stack-version: ">=2.1"
+#
+# Override the architecture used by stack, especially useful on Windows
+# arch: i386
+# arch: x86_64
+#
+# Extra directories used by stack for building
+# extra-include-dirs: [/path/to/dir]
+# extra-lib-dirs: [/path/to/dir]
+#
+# Allow a newer minor version of GHC than the snapshot specifies
+# compiler-check: newer-minor