tvl-depot/nix/stateMonad/default.nix

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# Simple state monad represented as
#
# stateMonad s a = s -> { state : s; value : a }
#
{ ... }:
rec {
#
# Monad
#
# Type: stateMonad s a -> (a -> stateMonad s b) -> stateMonad s b
bind = action: f: state:
let
afterAction = action state;
in
(f afterAction.value) afterAction.state;
# Type: stateMonad s a -> stateMonad s b -> stateMonad s b
after = action1: action2: state: action2 (action1 state).state;
# Type: stateMonad s (stateMonad s a) -> stateMonad s a
join = action: bind action (action': action');
# Type: [a] -> (a -> stateMonad s b) -> stateMonad s null
for_ = xs: f:
builtins.foldl'
(laterAction: x:
after (f x) laterAction
)
(pure null)
xs;
#
# Applicative
#
# Type: a -> stateMonad s a
pure = value: state: { inherit state value; };
# TODO(sterni): <*>, lift2, …
#
# Functor
#
# Type: (a -> b) -> stateMonad s a -> stateMonad s b
fmap = f: action: bind action (result: pure (f result));
#
# State Monad
#
# Type: (s -> s) -> stateMonad s null
modify = f: state: { value = null; state = f state; };
# Type: stateMonad s s
get = state: { value = state; inherit state; };
# Type: s -> stateMonad s null
set = new: modify (_: new);
# Type: str -> stateMonad set set.${str}
getAttr = attr: fmap (state: state.${attr}) get;
# Type: str -> (any -> any) -> stateMonad s null
modifyAttr = attr: f: modify (state: state // {
${attr} = f state.${attr};
});
# Type: str -> any -> stateMonad s null
setAttr = attr: value: modifyAttr attr (_: value);
# Type: s -> stateMonad s a -> a
run = state: action: (action state).value;
}