fix-let.patch

diff --git a/typechecker/src/inc/typechecker/Gather.scala b/typechecker/src/inc/typechecker/Gather.scala
index 4288483..e472854 100644
--- a/typechecker/src/inc/typechecker/Gather.scala
+++ b/typechecker/src/inc/typechecker/Gather.scala
@@ -46,6 +46,11 @@ class Gather(isTraceEnabled: Boolean) {
     }
   }
 
+  def substitute(env: Environment, subst: Substitution): Environment = {
+    if (subst.nonEmpty) scribe.trace(NL + "Apply substitution: " + Printer.print(subst))
+    env.mapValues(_.substitute(subst))
+  }
+
   def withTypeScheme(expr: Expr[NameWithPos], typ: TypeScheme): Infer[(Expr[NamePosType], List[Constraint])] = {
     val exprWithType = expr.map(_.withType(typ))
     Right((exprWithType, List.empty))
@@ -214,24 +219,35 @@ class Gather(isTraceEnabled: Boolean) {
     decl: TopLevelDeclaration[NameWithPos],
     env: Environment,
     source: String
-  ): Infer[(TopLevelDeclaration[NamePosType], List[Constraint])] =
+  ): Infer[(TopLevelDeclaration[NamePosType], List[Constraint])] = {
+    val solve = new Solve(isTraceEnabled)
+
     decl match {
       case let @ Let(name, expr, meta) =>
         gather(expr, env, source).flatMap {
           case (checkedExpr, constraints) =>
-            val eTp = checkedExpr.meta.typ.typ
-            val tp = TypeScheme.generalize(env, eTp)
+            val exprTp = checkedExpr.meta.typ.typ
 
-            if (tp.bound.nonEmpty)
-              scribe.trace(NL + "Generalize: " + tp.bound.map(Printer.print(_)).mkString("[", ", ", "]"))
+            trace(name, meta.pos, exprTp, source)
 
-            trace(name, meta.pos, tp, source)
+            solve.solve(constraints).map { subst =>
+              val updatedEnv = substitute(env, subst)
+              val updatedTp = exprTp.substitute(subst)
+              val tp = TypeScheme.generalize(updatedEnv, updatedTp)
 
-            val checkedLet = let.copy(binding = checkedExpr, meta = meta.withType(tp))
+              val checkedLet = let.copy(
+                binding = checkedExpr,
+                meta = meta.withType(tp)
+              )
 
-            Right((checkedLet, constraints))
+              if (tp.bound.nonEmpty)
+                scribe.trace(NL + "Generalize: " + tp.bound.map(Printer.print(_)).mkString("[", ", ", "]"))
+
+              (checkedLet, constraints)
+            }
         }
     }
+  }
 
   def gather(
     module: Module[NameWithPos],
diff --git a/typechecker/src/inc/typechecker/Solve.scala b/typechecker/src/inc/typechecker/Solve.scala
index 3696d7d..5b72912 100644
--- a/typechecker/src/inc/typechecker/Solve.scala
+++ b/typechecker/src/inc/typechecker/Solve.scala
@@ -18,7 +18,6 @@ class Solve(isTraceEnabled: Boolean) {
     ).replace()
   }
 
-  type Substitution = Map[TypeVariable, Type]
   val EmptySubst: Substitution = Map.empty
 
   def chainSubstitutions(ss: List[Substitution]): Substitution =
diff --git a/typechecker/src/inc/typechecker/package.scala b/typechecker/src/inc/typechecker/package.scala
index 6f268e0..4941d36 100644
--- a/typechecker/src/inc/typechecker/package.scala
+++ b/typechecker/src/inc/typechecker/package.scala
@@ -9,4 +9,5 @@ import scala.collection.immutable.{ List, Map }
 package object typechecker {
   type Infer[A] = Either[List[TypeError], A]
   type Environment = Map[String, TypeScheme]
+  type Substitution = Map[TypeVariable, Type]
 }