Idiomatic Nix bind helpers

bindAttrs

Like a bind-chain but operates over named attrset of required-effects.

bind.attrs { foo = 99; bar = pure 22; baz = asks (env: env.baz); }

Values that are non-effects become send params: send "foo" 99. Pass the optionalArg sentinel to mark a key as optional: bindAttrs probes via has-handler first and omits the key when no handler is installed (so a Nix function's default value can take over).

Result has same attr-keys with corresponding effect result.

See also: bind.comp, bind.fn for which this is the foundation.

NOTE: Ordering of chained effects.

Since an attrSet has no order, this function chains effects in same order as builtins.attrNames (alphabetical). If you need an special order for computations that might be order senstive, specify a __sort = names => names function.

bindComp

Turns a Nix effectful function into an effect chain via bindAttrs.

bindComp { bar = pure 22; } ({ foo, bar }: pure (foo * bar))

The function sees bar as the result of pure 22 and foo as the result of send "foo" false -- false comes directly from using lib.functionArgs f, the handler can know if "foo" is optional in f.

Optional args (those with defaults in the Nix function) are probed via has-handler before sending. If no handler exists, the arg is skipped and the Nix default kicks in.

This works by using bindAttrs on the intersection of function args and attrs.

bindFn

Like bindComp but works on normal Nix functions and turns its result into a pure-effect.

bindFn { bar = pure 22; } ({ foo, bar }: foo * bar)