dhall-text-shell
================

`dhall-text-shell` requires the expression to be `Text`.  But what if it was able to
also render expressions of type `(Text -> Text) -> Text`, and be given a shell
argument as the `Text -> Text` ?

```dhall
-- testfile.dhall
let text = https://raw.githubusercontent.com/dhall-lang/dhall-lang/v21.1.0/Prelude/Text/package.dhall
in  \(f : Text -> Text) -> text.concatMapSep "," Text f [ "hello", "world" ]
```

Would give:

```
$ dhall-text-shell --file testfile.dhall --argCmd cat
hello,world
$ dhall-text-shell --file testfile.dhall --argCmd "tr '[:lower:]' '[:upper:]'"
HELLO,WORLD
$ dhall-text-shell --file testfile.dhall --argCmd "pandoc -f markdown -t html"
<p>hello</p>
,<p>world</p>
$ dhall-text-shell --file testfile.dhall --argCmd "md5sum -z"
5d41402abc4b2a76b9719d911017c592  -,7d793037a0760186574b0282f2f435e7  -
```

Error messages:

```
$ dhall-text-shell --file testfile.dhall
Error: Expression doesn't match annotation

- Text
+ .. -> .. (a function type)
$ dhall-text-shell --file testfile.dhall --argCmd cat --argCmd cat
Error: Expression doesn't match annotation

- .. -> ..  (a function type)
+ Text
```

Supports multiple arguments as well:

```dhall
-- testfile2.dhall
let text = https://raw.githubusercontent.com/dhall-lang/dhall-lang/v21.1.0/Prelude/Text/package.dhall
in  \(f : Text -> Text) -> \(g : Text -> Text) -> text.concatMapSep "," Text f [ "hello", g "world" ]
```

```
$ dhall-text-shell --file testfile2.dhall --argCmd cat --argCmd "tr '[:lower:]' '[:upper:]'"
hello,WORLD
```

This is essentially a very minimal "FFI" for dhall, since it doesn't require
extending anything in the language.  It just requires you to parameterize your
program on that ffi function.

Note that for this to work meaningfully, your shell command must be "pure": it
must return the same stdout for any stdin, and shouldn't observably affect the
world every time it is run.