singletonListT

singletonListT is a helper used with the ListT monad transformer in functional programming. It creates a ListT computation that yields exactly one value, effectively lifting a plain value into the ListT context and preserving the effects of the underlying monad.

In many languages and libraries that define ListT, the type is ListT m a, a wrapper around m [a]. The typical type signature is singletonListT :: Monad m => a -> ListT m a, and a common implementation is singletonListT x = ListT (return [x]). Equivalently, it can be seen as ListT . return . (:[]), i.e., a transformer that wraps a single-element list within the underlying monad.

A simple example in Haskell notation would be: singletonListT 5, which yields a ListT m Int containing a single element 5. If the underlying monad is IO, executing the ListT value will perform IO effects and produce a list with the single element [5].

Usage and semantics: singletonListT is useful when composing ListT-based computations and you need to start from or inject a singleton result into the list-wrapped context. It allows uniform use of ListT combinators and ensures that the monadic effects of the underlying monad m are preserved while delivering exactly one value. This makes it convenient for constructing pipelines that may later produce multiple results, but begin with a single, known value.

See also: ListT, monad transformer, return, pure.