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Is there a function one can call within a Haskell program that does what :t does interactively? Or is this something Haskell cannot do since it's type would not make sense. What about a function that returned the name of the type as a String?
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Everything in Haskell has a type, so the compiler can reason quite a lot about your program before compiling it. Unlike Java or Pascal, Haskell has type inference. If we write a number, we don't have to tell Haskell it's a number.
If you are using an interactive Haskell prompt (like GHCi) you can type :t <expression> and that will give you the type of an expression. e.g. or e.g.
The shows functions return a function that prepends the output String to an existing String . This allows constant-time concatenation of results using function composition.
Function types Ordinary data types are for primitive data (like (Int) and (Char)) and basic data structures (like ([Int]) and ([Char])). Algebraic data types are types that combine other types either as records ('products'), e.g. or as variants ('sums'), e.g.
The question can be understood twofold:
can a polymorphic function inquire the concrete type it is called with. The other questions describe how it could be done.
can we get to runtime statically known information about a binding. This is what actually the :t does. There are ways to get it:
a. to just print it during compilation, you could enable PartialSignatures extension, and add to arbitrary expression signature :: _, then its type will be printed during compilation.
b. to actually get the type as a data for runtime handling, you could use TemplateHaskell extension and function reify from it.
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