It's an idea that comes from Smalltalk, where for the longest time they've had a way to search for object methods given an input and an output, and it would sea

It's an idea that comes from Smalltalk, where for the longest time they've had a way to search for object methods given an input and an output, and it would search which object method did that. So you can give it "blah" and "Blah" and it would find the method "capitalize" for instance. Hoogle is like that but for Haskell, and instead of giving inputs and outputs you give it a few types and it looks for them in the function signatures.