Mitchell Documentation

Copyright 2019 Systems & Technology Research, LLC and other contributors.

Distribution Statement A – Approved for Public Release, Distribution Unlimited

This site uses Just the Docs, a documentation theme for Jekyll.

Search

Reading Compiler Errors in Mitchell

The following is a list of common errors that occur when compiling programs and descriptions of how to understand them.

Function is not of arrow type.

Function not of arrow type.
  function: [string]
  in: "hello world" print

This message means that somewhere in the code there is something that is not a function being used as one. The error message gives the inferred type of the thing that was used as a function (string in the compiler above), as well as the snippet of code where the problem is.

In the example above, the snippet of code has the arguments backwards, but this isn’t as silly of an example as it might seem. A common error is to write something like the following

val x = "hello world"
print x

That code will result in the error above, because it is interpreted by the compiler the same as writing the following, which looks like applying a string to the print function.

val x = ("hello world" print) x

The fix is to make sure to include the val assignment for every statement.

val x = "hello world"
val _ = print x

The underscore _ is a way of discarding the non-useful result of the call to print.

Undefined variable: x.

Undefined variable: x.

This message means that there is a variable, here named “x”, that is used but not defined anywhere. In Mitchell, variables have to be defined before they can be used. This means that, for example, you have to define functions before you use them in the file.

If you get this message about a function from the Standard Basis Library, make sure that

  • the function name is spelled correctly, including being the correct case, and
  • the function is part of the module that you are referring to, not some other module.

For example, if you refer to String.splitl, you will get the message

Undefined variable: String.splitl.

that is because splitl is part of the substring module, not the string module. if instead there was a typo and substring was written as substirng, the error message would be

undefined structure: Substirng.

undefined structure: X

undefined structure: X.

This means that you are referring to something that is part of a module, but the module name was not recognized. Module names are case sensitive, and some modules are very similar to each other in name and content (like String and Substring), so make sure you are using the right one.