FP Use Cases

Each row in the table below represents a “developer need“ which has been expressed through one or more issues in this repository. For each need, we record precedents addressing this need, and the way that the Function Helpers proposals currently addresses the need. Having the needs listed here is just a way to organize information for the facilitation of clear and informed communication.

The addition of a “need“ to the table does not necessarily mean that addressing it (better or more directly) is within the scope of this proposal. If a need has currently been deemed out of scope of the proposal, we will use strike through to indicate it.

Developer Need	Proposed Solution	Precedents
To apply a series of unary functions to a value in left-to-right order	Function.pipe(value, ...fns)	pipe in fp-ts |> in F# |> in Elixir |> in Elm |> in Julia |> in LiveScript Data.Function.& in Haskell
To apply a series of unary async functions to a value in left-to-right order	Function.pipeAsync(value, ...fns)
To define a function in terms of a series of function calls in left-to-right order	Function.flow(...fns)	flow in Lodash pipe in Ramda flow in fp-ts pipe(fns) in Sanctuary
To define a function in terms of a series of function calls in right-to-left order	Function.flow(...fns.reverse())	compose in Ramda flowRight in Lodash
To define a function in terms of a series of async function calls in left-to-right order	Function.flowAsync(...fns)	pipeP in Ramda p-pipe by sindresorhus
To define a function in terms of a series of async function calls in right-to-left order	Function.flowAsync(...fns.reverse())	composeP in Ramda
To (partially) compose two unary functions in right-to-left order [1]	f => g => x => f(g(x))	compose in Sanctuary map in Sanctuary map in Ramda B from combinatory logic . in Haskell
To create a function that always returns the same value	Function.constant(value)	constant in lodash always in Ramda constant in fp-ts K in Sanctuary constant-function in stdlib K from combinatory logic const in Haskell
A function that returns its input unchanged to be used as an argument to HOFs	Function.identity	identity in lodash identity in Ramda identity in fp-ts I in Sanctuary identity-function in stdlib I from combinatory logic id in Haskell
A way to run side effects inside a functional pipeline	Function.aside(sideEffect)	tap in Ramda tap in lodash inspect in Rust Debug.Trace on Hackage