Trying to Define `$` Type

问题

Using Standard ML of New Jersey (v110.77), I'm trying to define the $ data type from Purely Functional Data Structures:

datatype alpha susp = $ of alpha

But I get an error:

- datatype alpha susp = $ of alpha;
stdIn:1.11-2.7 Error: syntax error: deleting  ID ID EQUALOP

What am I doing wrong?

回答1:

In Standard ML, type variables are prefixed with a single quote:

datatype 'alpha susp = $ of 'alpha

来源：https://stackoverflow.com/questions/28802305/trying-to-define-type