I wrote an executable, ghcscriptrender, that takes a Haskell script and can render it in html, including the outputs. There are other options than html. This executable is useful to write some Haskell code with rmarkdown.

First you have to define an engine. In the first chunk of your knitr document:

knitr::knit_engines$set(
    ghc = function (options) 
      {
        engine = options$engine
        f = basename(tempfile(engine, ".", ".txt"))
        writeLines(options$code, f)
        on.exit(unlink(f))
        code = paste(f, options$engine.opts)
        cmd = options$engine.path 
        out = if (options$eval) {
          message("running: ", cmd, " ", code)
          tryCatch(system2(cmd, code, stdout=TRUE, stderr=FALSE), 
                   error = function(e) {
                          if (!options$error)
                            stop(e)
                            paste("Error in running command", cmd)
                          })
        }
        else ""
        if (!options$error && !is.null(attr(out, "status"))) 
          stop(paste(out, collapse = "\n"))
        knitr::engine_output(options, options$code, out)
      }
)

Then follow this example, whose rendering is shown below.