GHCi "let" -- what does it do?

While programming in GHCi, you're like programming in the IO monad with do syntax, so for example you can directly execute an IO action, or use monadic bind syntax like r <- someIOFun.

let is also a part of do so you can also use this. I think it's being desugared into let .. in <rest of the computation>, so for example when you do this:

ghci> let a = 1
ghci> someFun
ghci> someFun2

It's like:

let a = 1 in
do someFun
   someFun2

Here's the relevant part of the documentation.

GHCI statements are executed as IO computation. So let is same as what you have inside an IO monad where you bind non-IO expression using let.

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.