---
title: "Writing Haskell in knitr"
date: "2016-08-28"
output: html_document
---

```{r setup, include=FALSE}
knitr::opts_chunk$set(echo = TRUE)
```

I wrote an executable, [ghcscriptrender](https://github.com/stla/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](https://raw.githubusercontent.com/stla/ghcscriptrender/master/tests/knitr_ghc_demo.Rmd), whose rendering is shown below.

<iframe style="height:1600px; width:100%; overflow:hidden; padding:0; border:1;"  src="http://htmlpreview.github.io/?https://raw.githubusercontent.com/stla/ghcscriptrender/master/tests/knitr_ghc_demo.html"></iframe>