Search code examples
haskellghcilet

GHCi "let" -- what does it do?

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" (without "in") is not part of the Haskell language per se, and on the other hand, it doesn't appear to be a GHCi command either, as it's not prefixed by colon.

Solution

  • 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