A simple implementation of John Launchbury's operational semantics for lazy languages.
Based on John Launchbury's paper A natural semantics for lazy evaluation.
In GHCI, you can use displayReduce to trace through the evaluation of expressions:
*TinyLaunchbury> displayReduce simpleExpr
Reducing let: let u = 3 + 2, v = u + 1 in v + v : {}
|Reducing primitive: v + v : {v -> u + 1, u -> 3 + 2}
||Reducing variable: v : {v -> u + 1, u -> 3 + 2}
|||Reducing primitive: u + 1 : {u -> 3 + 2}
||||Reducing variable: u : {u -> 3 + 2}
|||||Reducing primitive: 3 + 2 : {}
||||||Returning constructor: 3 : {}
||||||Returning constructor: 2 : {}
|||||Primitive evaluated to 5
||||Rebinding var u to 5
||||Returning constructor: 1 : {u -> 5}
|||Primitive evaluated to 6
||Rebinding var v to 6
||Reducing variable: v : {v -> 6, u -> 5}
|||Returning constructor: 6 : {u -> 5}
||Rebinding var v to 6
|Primitive evaluated to 12
Ans: 12