Js char io (#2887)

* Implement `{get,put}Char` for javascript backend

* Update changelog

CHANGELOG.md

@@ -29,10 +29,11 @@
 * Non-recursive top-level constants are compiled to eagerly evaluated
   constants in Chez Scheme.
 
-#### Node.js
+#### Node.js/Browser
 
 * Generated JavaScript files now include a shebang when using the Node.js backend
 * NodeJS now supports `popen`/`pclose` for the `Read` mode.
+* `getChar` is now supported on Node.js and `putChar` is supported on both JavaScript backends.
 
 ### Compiler changes
 

libs/prelude/Prelude/IO.idr

@@ -75,8 +75,12 @@ export
 prim__getString : Ptr String -> String
 
 %foreign "C:putchar,libc 6"
+         "node:lambda:x=>process.stdout.write(x)"
+         "browser:lambda:x=>console.log(x)"
 prim__putChar : Char -> (1 x : %World) -> IORes ()
+
 %foreign "C:getchar,libc 6"
+         "node:support:getChar,support_system_file"
 %extern prim__getChar : (1 x : %World) -> IORes Char
 
 %foreign "C:idris2_getStr, libidris2_support, idris_support.h"

support/js/support_system_file.js

@@ -62,6 +62,16 @@ function support_system_file_getStr () {
   return support_system_file_readLine({ fd: 0, buffer: Buffer.alloc(0), name: '<stdin>', eof: false })
 }
 
+function support_system_file_getChar() {
+  const readBuf = Buffer.alloc(1);
+  if (support_system_file_fs.readSync(process.stdin.fd, readBuf, 0, 1) === 0) {
+    // No bytes read, getChar from libc returns -1 in this case.
+    return String.fromCharCode(-1)
+  } else {
+    return readBuf.toString('utf-8')
+  }
+}
+
 function support_system_file_parseMode(mode) {
   return mode.replace('b', '')
 }