Debug

Turn any kind of value into a string.

toString 42 == "42"

toString [ 1, 2 ] == "[1,2]"

toString ( 'a', "cat", 13 ) == "('a', \"cat\", 13)"

toString "he said, \"hi\"" == "\"he said, \\\"hi\\\"\""

Notice that with strings, this is not the identity function. It escapes characters so if you say Html.text (toString "he said, \"hi\"") it will show "he said, \"hi\"" rather than he said, "hi". This makes it nice for viewing Gren data structures.

Note: This is not available with gren make --optimize which gets rid of a bunch of runtime metadata. For example, it shortens record field names, and we need that info to toString the value! As a consequence, packages cannot use toString because they may be used in --optimize mode.

Log a tagged value on the developer console, and then return the value.

1 + log "number" 1 -- equals 2, logs "number: 1"

length (log "start" []) -- equals 0, logs "start: []"

It is often possible to sprinkle this around to see if values are what you expect. It is kind of old-school to do it this way, but it works!

Note: This is not available with gren make --optimize because (1) it relies on toString which has the same restriction and (2) it is not a pure function and would therefore have unpredictable behavior when paired with compiler optimizations that move code around.

Note: If you want to create a terminal application that prints stuff out, use ports for now. That will give you full access to reading and writing in the terminal. We may have a package in Gren for this someday, but browser applications are the primary focus of platform development for now.

This is a placeholder for code that you will write later.

For example, if you are working with a large union type and have partially completed a case expression, it may make sense to do this:

type Entity = Ship | Fish | Captain | Seagull

drawEntity entity =
  case entity of
    Ship ->
      ...

    Fish ->
      ...

    _ ->
      Debug.todo "handle Captain and Seagull"

The Gren compiler recognizes each Debug.todo so if you run into it, you get an uncatchable runtime exception that includes the module name and line number.

Note: This is not available with gren make --optimize or packages. The idea is that a todo can be useful during development, but uncatchable runtime exceptions should not appear in the resulting applications.

Note: For the equivalent of try/catch error handling in Gren, use modules like Maybe and Result which guarantee that no error goes unhandled!