Typing expressions
In the Expressions DSL, types are not written explicitly by the programmer. However, due to the simple nature of our expressions, we can easily deduce the type of an expression by looking at its shape. In this DSL we have a fixed set of types: string, integer, and boolean. The mechanism of deducing a type for an expression is usually called type computation or type inference.
The base cases for type computation in the Expressions DSL are constants; trivially, an integer constant has type integer, a string constant has type string, and a boolean constant has type boolean.
As for composed expressions, besides computing a type, we must also check that its sub-expressions are correct with respect to types. This mechanism is usually ...
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