GHCi “let” — what does it do?

Question

I\'d appreciate if someone could point to docs on what "let" does in GHCi, or failing that, explain it convincingly.

So far as I can tell, "let" (

Answer 1

For more down-to-the-code details, this comment in TcRnDriver.lhs might be enlighting:

--------------------------------------------------------------------------
                Typechecking Stmts in GHCi

Here is the grand plan, implemented in tcUserStmt

        What you type The IO [HValue] that hscStmt returns
        ------------- ------------------------------------
        let pat = expr ==> let pat = expr in return [coerce HVal x, coerce HVal y, ...]
                                        bindings: [x,y,...]

        pat <- expr ==> expr >>= \ pat -> return [coerce HVal x, coerce HVal y, ...]
                                        bindings: [x,y,...]

        expr (of IO type) ==> expr >>= \ it -> return [coerce HVal it]
          [NB: result not printed] bindings: [it]

        expr (of non-IO type, ==> let it = expr in print it >> return [coerce HVal it]
          result showable) bindings: [it]

        expr (of non-IO type,
          result not showable) ==> error

So a command at the GHCi prompt can have up to three effects: Some code is evaluated, some things are printed, and some variable names are bound. Your case (the first one in the comment) binds variables, but does not print.

The syntax does resemble the do-notation, so @sinan’s answer is somewhat right, but it is not really what is happening under the hood – otherwise, for example, nothing would ever be printed.