GHCi “let” — what does it do?

I'd appreciate is 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.

Answers


While programming in GHCi, you're like programming in IO monad with do syntax, so for example you can directly call a function with type of IO a, 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

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